Retell AI Phone Number Setup

Before an agent can answer or place a single call, it needs a phone number bound to it, and Retell gives you two genuinely different ways to get one: buy a number directly through Retell, or import an existing number from a telephony provider you already use, most commonly Twilio, Telnyx, or Vonage, via SIP trunking. Which path makes sense depends heavily on whether you are starting fresh or already have infrastructure and numbers you need to keep.

I handle this setup step on essentially every Retell project, and it is worth getting the buy-versus-import decision right at the start, since switching later means either porting a number or changing what your business publishes as its contact line. This post covers both paths, the SIP trunking mechanics for imported numbers, and how numbers get bound to agents for inbound and outbound handling.

How Retell AI Phone Numbers Work

A phone number in Retell is bound to a specific agent, or two, one for inbound calls and a separate one for outbound, determining how calls to or from that number are handled. This binding is what connects the telephony layer, the actual phone number and carrier routing, to the AI logic that answers or places the call.

Buying a Number

For a business starting fresh with no existing number that needs to carry over, purchasing a number directly through Retell is the simplest path, with no separate telephony provider account or SIP configuration required. This works well for a new voice agent line, a dedicated number for a specific campaign, or any situation where publishing a new number is not a problem.

Porting an Existing Number

If your business already has a published number customers know and call, replacing it outright is rarely acceptable. Two options handle this: porting the number entirely into Twilio (or another supported carrier) and then importing it into Retell via SIP trunking, or forwarding calls from the existing number to a new Retell-connected number as a lower-friction interim step while a full port is in progress. Porting typically takes a meaningful stretch of business days depending on the carrier and country, so plan for that timeline rather than assuming it happens instantly, and forwarding is worth using as a bridge if you need the agent live before the port completes.

Inbound vs Outbound Numbers

A number can be configured for inbound only, outbound only, or both, with a separate agent bound to each direction if needed. Leaving the inbound agent unset on a number dedicated purely to outbound calling is a deliberate choice worth making explicitly, since it prevents the number from accidentally accepting inbound callbacks it was never meant to handle, the same consideration covered from the calling side in Retell AI Outbound Calls.

Caller ID and Call Routing

For imported numbers connected via SIP trunking, elastic SIP trunking is Retell's recommended integration method, supported by major providers including Twilio, Telnyx, and Vonage. The setup involves configuring termination and origination URIs on your telephony provider's side to route calls through Retell's SIP server, along with authentication, typically credential-based rather than IP-based, since Retell's SIP server does not have a static IP to whitelist against.

If your provider does not support elastic SIP trunking, or your telephony setup is more complex than a straightforward number-to-agent mapping, Retell also supports a Dial to SIP URI method: your own system calls Retell's Register Phone Call API to get a call ID, then dials a SIP URI constructed from that ID within a five-minute window. This method gives you more control over call routing logic but does not support Retell's built-in call transfer feature, the pattern covered in Retell AI Call Transfer, meaning transfers need to be implemented separately on your own infrastructure if you need this method specifically.

Number Configuration

Once a number is purchased or imported, binding it to an agent, or two separate agents for inbound and outbound, from the Phone Numbers section of the Retell dashboard is what actually connects the telephony layer to your configured agent logic. Retell does not validate a SIP configuration at the moment you save it, the connection is only actually tested when a real call is placed, so an apparently successful save is not confirmation the setup is correct end to end.

Testing Your Number

Always place a real test call after configuring a number, whether newly purchased or freshly imported via SIP, rather than trusting the dashboard's saved state alone. For an imported number specifically, call the number from an external line entirely outside your own network and confirm it actually reaches the bound Retell agent, since this is the step that surfaces SIP misconfigurations, wrong termination URIs, authentication mismatches, that a dashboard save will not catch on its own.

Common Phone Number Problems

Call rings then immediately disconnects. Typically a SIP trunk authentication or IP configuration issue on the telephony provider's side, worth checking against Retell's published IP block and current SIP endpoint details, or a mismatch on the termination URI configuration.

Number imports successfully but calls never reach the agent. Check that the number is actually bound to an agent in the Phone Numbers section, and confirm both origination and termination URIs on the provider side are pointed at the correct Retell SIP endpoint for your chosen transport protocol.

International numbers not behaving as expected. For Retell-purchased numbers, international calling support depends on the specific country. For imported numbers, international calling capability depends entirely on your telephony provider's own settings and coverage rather than anything Retell controls.

Getting phone number setup right, whether that is a straightforward purchase or a SIP trunk import preserving an existing published number, is exactly the kind of Retell AI infrastructure work I take on for clients. Book a free 30-minute call and bring your current phone setup, and we will get it connected correctly.

Frequently Asked Questions

Does Retell AI provide phone numbers?

Yes, you can purchase a number directly through Retell for a simple, self-contained setup, or import an existing number from a telephony provider like Twilio, Telnyx, or Vonage via SIP trunking if you need to preserve a number your business already publishes.

Can I port my existing business number to Retell AI?

Yes, typically by porting the number into a supported carrier like Twilio first and then importing it into Retell via SIP trunking, or using call forwarding as a lower-friction interim step while a full port is in progress.

Can Retell AI receive inbound calls?

Yes, once a number is bound to an inbound-capable agent in the Phone Numbers section of the dashboard, calls to that number are routed to the agent automatically.

Can Retell AI make outbound calls?

Yes, once a number is bound to an outbound-capable agent, calls can be triggered through the Create Phone Call API, either individually or through Retell's Batch Calling feature for running a campaign across a list of contacts.

How much does a Retell phone number cost?

Pricing for both directly purchased and imported numbers should be confirmed on Retell's current pricing page, since exact costs and any per-minute usage charges can change over time.

Can I use my own phone number with Retell?

Yes, through SIP trunking, either elastic SIP trunking if your telephony provider supports it, or the Dial to SIP URI method for more complex or unsupported setups, though the latter does not support Retell's built-in call transfer feature.

If you would rather have this set up than set it up yourself, I take on Retell AI voice agent work through Upwork.