Best AI Voice Agent Platforms for VoIP Resellers and MSPs (2026): SIP Trunks, Call Transfer and White-Label Compared

If you run a VoIP reseller or MSP, pick the platform on its telephony first and its price second. Four questions decide it: which SIP connection modes it documents, how it hands a call back to a human, whether you keep your own numbers, and whether it can be white-labeled per client. In the pages we read in November 2026, Bland documents the deepest SIP behavior, but none of its pages we read mention white-label. Retell and Vapi publish pay-as-you-go prices, but their pricing pages mention no white-label or reseller terms. PulsyAI, VoiceAIWrapper and Synthflow cover white-label in different ways. Details and sources are below.

Disclosure: PulsyAI publishes this post and sells a competing product. We list ourselves because we are one of the options, and we say where others fit better. Everything about other vendors was read on their own pages between 3 and 4 November 2026, and each claim links to its source. Where a page says nothing, we write 'not documented'. That means we did not find it on the pages we read, not that the feature is missing.

Key Takeaways

• Registration trunks matter if your PBX or UCaaS has no static IP. In the pages we read, only Bland and PulsyAI document SIP registration. Retell documents username and password fields but not registration.
• SIP REFER hands the call back to your carrier or PBX and frees the AI leg. It only works if your side supports REFER. Dial (bridged) transfer works anywhere, but the platform stays in the call.
• Pay-as-you-go prices run from $0.05 to $0.14 per minute on the platforms that publish them, but the scope of each price differs, so compare what is included.
• For per-client white-label with Stripe billing, the realistic shortlist in this set is PulsyAI, VoiceAIWrapper and Synthflow (by contract). The others publish no white-label terms on the pages we read.
• Numbers bought inside PulsyAI are inbound-only. Outbound calling needs imported Twilio, Telnyx or SIP numbers.

How we ranked these platforms

We ranked for one reader: the owner or technical lead of a VoIP reseller, hosted-PBX provider or MSP who wants AI agents on their own carrier and numbers, under their own brand. We did not build a single score. A score hides the fact that the right answer depends on your PBX. Instead we checked eight criteria on each vendor's own pages:

1. SIP connection modes: IP-authenticated trunk, registration trunk, or acting as a SIP client to your PBX.
2. BYOC and number import: can you keep your carrier and your numbers?
3. Transfer methods: SIP REFER, new INVITE (bridged), warm or cold.
4. IVR and DTMF: keypad input in the same call.
5. Multi-trunk: different carriers per client or region.
6. Per-client white-label and billing.
7. Multi-tier reselling: can your resellers resell?
8. Pricing model, dated and linked.

The order below is our judgment of fit for this reader. It is not a measure of voice quality or call-handling accuracy, which we did not test. We list PulsyAI first because white-label plus the three SIP modes is the combination this reader asked for. You should weigh that against the fact that we wrote this post.

Three telephony terms, in plain words

IP trunk vs registration trunk vs SIP client

These are three ways to connect an AI platform to your phone system.

• IP (static IP) trunk. Your carrier or session border controller (SBC) trusts the platform's IP addresses, and the platform trusts yours. No password is exchanged. It fits a fixed SBC with a static address. Bland's docs call this the default and say there are 'no credentials to manage', but also warn that a firewall rule alone is not enough (Bland SIP docs).
• Registration trunk. The platform logs in to your server with a username and password (SIP REGISTER), the way a desk phone does. It fits systems that cannot allow-list outside IPs or have no static IP.
• Registrar / SIP-client mode. The platform connects as a client to a hosted PBX or cloud phone provider, as if it were one more extension. It fits hosted PBX and UCaaS where you cannot configure a trunk on your side.

Rule of thumb: fixed SBC means IP trunk. Hosted PBX or UCaaS without a static IP means registration or client mode.

SIP REFER vs dial (bridged) transfer

When the AI agent hands a caller to a person, there are two basic methods.

• SIP REFER. The agent tells your carrier or PBX to take over the call. RFC 3515 defines REFER as a method that 'asks the receiving agent to contact a resource' and lets the sender be notified of the outcome (RFC 3515). The AI platform then leaves the call. The tradeoff: your carrier or PBX must support REFER. Bland's docs say REFER hand-off needs a call that arrived over SIP and a PBX that supports REFER. Retell says you must configure your trunking provider to allow SIP REFER and PSTN transfer (Retell docs).
• Dial (bridged) transfer. The platform places a new call to the human and bridges the two. It works with nearly any setup. Synthflow's docs say SIP INVITE is 'universally supported' and suggest warm mode if a PBX rejects REFER (Synthflow docs). The tradeoff: the platform stays in the call, so both legs stay up. Bland lists a separate transfer-time rate for this ($0.05 per minute on its Start plan, as of November 2026), which shows why the leg matters for cost.

Plain choice: if your PBX supports REFER, use it and free the AI leg. If you are not sure, start with dial transfer, which works everywhere, and test REFER later.

1. PulsyAI: best for white-label with all three SIP modes

What it is. A white-label AI voice platform for agencies, MSPs and VoIP providers. We build it.

SIP modes. Our SIP trunk page lists Static IP, Registration (gateway, username, password) and Registrar / SIP client mode for hosted PBX or cloud providers. Each supports inbound forwarding and outbound gateway setup.

Transfer. Dial and SIP REFER. Dial is the default for new SIP trunks.

Numbers and carriers. BYOC, with number import from Twilio, Telnyx or your own SIP carrier. The page says you keep your carrier contracts and billing. Numbers bought inside PulsyAI are inbound-only; outbound calling needs imported numbers.

Also listed. IVR with DTMF, and multi-trunk management so different clients can use different carriers. See the VoIP page for the provider view.

White-label and reselling. Custom domain, logo and colors, Stripe Connect so client payments go to your account, and CDR and billing export per client, agent and minute. A multi-tier white-label option is on the Enterprise plan, and the MSP page covers the PSA integrations.

Price (as of November 2026). The features page shows the White-Label plan at $99 per month with 1,000 minutes included, labeled a limited-time offer. Starter is pay-as-you-go at a flat $0.10 per minute. Enterprise is custom. At full use of the 1,000 included minutes, $99 divided by 1,000 is $0.099 per minute. That is a platform fee, not a full cost of a call.

Limits. We have not published which PBX or switch brands we have connected, so we do not list any. Test your own PBX in a trial before you commit. Our features page says the free trial needs no credit card, but does not state its length.

2. Bland: best documented SIP behavior for a technical team

What it is. A developer-oriented voice agent platform.

SIP modes. The SIP integration docs describe IP authentication as the default, plus registration: Bland registers with your server using a username and password via SIP REGISTER. Optional credentials can also go on SIP INVITE requests.

Transfer. The most detailed we found. A Transfer Call node can send a SIP REFER hand-off (needs a call that arrived over SIP and a PBX that supports REFER), a new INVITE where Bland stays in the call and bridges audio, or a warm transfer with a consult call.

Numbers. The docs mention number porting. Its pricing page FAQ says customers who bring their own telephony pay carriers directly and do not pay transfer fees.

White-label. The pricing page does not mention white-label or reseller programs.

Price (as of November 2026). Start: $0.14 per minute talk time and $0.05 per minute transfer time, $0 platform fee. Build: $0.12 and $0.04 with a $299 monthly platform fee. Enterprise: custom. Arithmetic: Build saves $0.02 per talk minute, so it beats Start only above $299 / $0.02 = 14,950 talk minutes a month, before transfer time.

Fit. A strong pick if you have engineers and one internal use case. A weak fit if you need to resell under your brand, because we found no white-label terms.

3. Retell AI: best pay-as-you-go option with documented REFER

SIP modes. The custom telephony docs cover IP allow-listing and username and password fields. They do not mention SIP registration, so registration is not documented there. Twilio, Telnyx and Vonage have setup guides, and other SIP trunk providers are expected to work. Guides for Avaya, Genesys Cloud, Five9 and Amazon Connect are listed too.

Transfer. The docs support transfer with SIP trunking. For a cold transfer using REFER, your trunking provider must allow SIP REFER and PSTN transfer.

Price (as of November 2026). The pricing page shows $0.07 to $0.31 per minute for voice agents on pay-as-you-go, with a calculator example of $0.11. It lists 'No Charge for Sip Trunking/Custom Telephony' and no charge for custom numbers. Extra concurrency is $8 per concurrency per month.

White-label. Not mentioned. It lists a Solution Partner Program but not its terms.

Fit. Good for a reseller who builds their own wrapper or serves a few direct clients. Not a white-label portal.

4. Vapi: best for developers building their own front end

SIP modes. The SIP trunk docs show username and password credentials for a custom trunk and list Plivo, Telnyx, Zadarma and custom providers. They say 'We generally don't recommend IP-based authentication for SIP trunks as it can lead to routing issues.' SIP registration is not documented there.

Transfer. The call forwarding docs list blind and warm transfer modes. Several warm modes are Twilio only. SIP REFER is not documented as a transfer mode; it appears only in troubleshooting.

Price (as of November 2026). The pricing page shows $0.05 per minute Vapi hosting, with model costs billed at provider cost. Core is $29 per month. Pro is 10% of hosting with a $999 monthly minimum. Vapi telephony and SIP are listed as free, and Twilio and Telnyx carrier rates are charged by the provider. Arithmetic for 10,000 minutes: 10,000 x $0.05 = $500 hosting, before model, voice and carrier costs.

White-label. Not mentioned on the pricing page.

Fit. Best when you write code and want control over each component. For a PBX that needs REFER, check with Vapi support first.

5. Synthflow: best for enterprise contracts, including BPO white-label

Price (as of November 2026). The pricing page says new customers buy one enterprise contract scoped to call volume, concurrency, telephony and integrations, with no free plan, trial or per-minute rate. White-label has no listed price; partners such as BPOs and software platforms can include it in their contract. Older third-party figures for its white-label add-on are therefore not confirmed on this page.

SIP and transfer. Its docs say cold transfers use SIP REFER, while warm transfers create a new SIP INVITE leg. They note that Vonage and Aircall do not support REFER for mid-call transfers. The Aircall page says SIP trunking is only available on the Enterprise plan. Registration trunks are not documented in the pages we read.

Fit. A fit if you can run a sales cycle and a contract. Not a fit if you want to sign up and launch a first client this week.

6. VoiceAIWrapper: best low-cost white-label shell over other engines

VoiceAIWrapper is a white-label layer. Its site says you can create agents under your own brand, give each client a branded portal, and set your own prices. Its native engine uses technology from providers including ElevenLabs and OpenAI. You can also connect your own Vapi, Retell or ElevenLabs keys. It says you can connect VoIP or SIP telephony from providers such as Twilio, Vonage, Telnyx and Zadarma.

Price (as of November 2026). On its pricing page: Pay as You Go is $0 to start; Growth is $79 per month with 3 client accounts; Scale is $499 per month with 20 client accounts. White-label is not explicitly listed in Scale's feature list, so confirm that with the vendor.

Limits. SIP modes, registration and REFER are not documented on the pages we read, so test your PBX path before you sell it. Telephony behavior also depends on which engine sits underneath.

7. Telnyx Voice AI: best if you already run on Telnyx

Telnyx is a carrier first. Its AI assistant pricing shows the voice engine at $0.05 per minute, covering orchestration, speech-to-text and text-to-speech. LLM tokens are billed separately, telephony starts at $0.0032 per minute, and the page says a production agent lands near $0.06 per minute. Its footnote says estimates exclude rounding to 60-second increments. A release note says a SIP Refer tool can be added to an AI Assistant to hand off active calls to another provider. White-label is not mentioned on the pricing page. The pricing page and release note do not mention registration trunks.

Fit. A fit if your numbers and SIP trunks are already on Telnyx. As a reseller product you would need a billing and branding layer on top.

Comparison table (as of November 2026)

'Not documented' means we did not find it on the pages cited in this post. Ask the vendor before you assume either way. Prices are not like for like: scope differs, and some exclude LLM or carrier costs.

How to choose

1. Start with your PBX. Fixed SBC with a static IP: any platform that supports IP trunks. Hosted PBX or UCaaS without a static IP: confirm registration or SIP-client mode in writing.
2. Test transfer both ways. Run a dial transfer and a REFER transfer against your own system before you promise anything to a client.
3. Decide whether you resell or build. If clients should log in under your brand and pay you through Stripe, you need white-label. If you build one internal agent, a developer platform is fine.
4. Keep your numbers. Check number import. A number bought inside a platform may not be portable or may not support outbound calls. On PulsyAI, in-app numbers are inbound-only.
5. Price from your own minutes. Multiply your expected monthly minutes by the per-minute rate, add the platform fee and carrier cost, and compare. The real cost of a voice AI minute post breaks down the layers.

For the carrier side, see our Twilio SIP trunk setup guide. For MSP packaging, see adding managed AI answering to an MSP offering. For a wider white-label comparison, read white-label AI voice agent platforms compared.

FAQ

Do I need a registration trunk or an IP trunk?

Use an IP trunk if you have a fixed SBC with a static IP. Use a registration trunk or SIP-client mode if your PBX is hosted or you cannot allow-list outside IPs.

Is SIP REFER better than dial transfer?

REFER frees the AI leg, but needs REFER support on your carrier or PBX. Dial works almost anywhere, but both legs stay up. Test both.

Can I keep my existing numbers?

PulsyAI lists number import from Twilio, Telnyx or your own SIP carrier. For the others, check their docs; we did not verify number import for every vendor.

These prices and features change. We read every source above in November 2026, so re-check the linked pages before you commit. To see the SIP modes on your own PBX, start from our SIP trunk page.