The Outbound Voice AI Phone Number Checklist

Most outbound voice AI pilots stall on the phone number, not the agent. Nobody can say which number the agent should call from, whether IT has to open the firewall, or why the first test calls show up as Spam Likely. This checklist puts those decisions in launch order, with lead times.

Should you buy new numbers or bring your own?

Buy new numbers if you need US or Canadian numbers quickly and want caller reputation tools built in. Bring your own if customers already know your numbers or your telecom team needs calls to stay on your current carrier. You can also do both.

A bought number stays yours until you release it. Add a payment method to the workspace before buying. Voice agent usage is billed per minute on top of telephony either way. The pricing page lists current rates.

To bring your own, point your carrier's elastic SIP trunk at Retell, then import the number with your carrier's termination URI. The step-by-step setup, with notes for Twilio, Telnyx and Vonage, is in how to keep your existing phone number using a SIP connection. If your carrier cannot do elastic trunking, use dial to SIP URI: your system registers each call through the API and dials it to Retell, but Retell cannot then transfer or end calls on your carrier.

What if your numbers live in a contact center or UCaaS platform?

If the platform can route those numbers over a SIP trunk to an external SIP address, they work like any bring-your-own number. Retell publishes setup guides for Five9, Genesys Cloud, Avaya and Amazon Connect.

Retell's docs have no dedicated guide for UCaaS platforms such as Zoom Phone, so ask your admin or vendor:

• Can the platform send calls for this number to an external SIP destination, and receive calls back over the same trunk?
• Will it let an outside system place outbound calls that present this number as caller ID?
• Can we allowlist the far end's IP ranges and open its media ports?
• If not, can our platform dial a per-call SIP URI that our own code supplies?

If every answer is no, buy a new Retell number for outbound and leave the main line where it is.

Which countries can you call, and from which numbers?

You can only buy Retell-managed numbers in the US and Canada, but Retell's Twilio numbers can call 15 countries. Those 15 destinations are the US, Canada, Mexico, the UK, Germany, France, Spain, Italy, India, Australia, Japan, Indonesia, the Philippines, Malaysia and Thailand. Per-minute rates range from $0.015 for the US to $0.80 for the Philippines. On Telnyx, Retell numbers call the US, Canada and India.

For a mostly US program, buy US numbers in Retell with area codes that match your audience. If you need a local caller ID in another country, or you need to reach a country outside that list, import a number from a carrier that serves it. With imported numbers, which countries you can call depends on your carrier's settings. On Twilio trunks, for example, you enable each country under the trunk's geographic permissions.

Verified phone numbers, branded call and A2P SMS registration are all US-only today.

How do you keep outbound calls from showing as Spam Likely?

Register the number, show your business name, and ramp volume slowly. Carriers flag numbers that were tied to spam before being recycled, numbers whose call volume spikes with no warm-up, and numbers with low pickup rates.

• Business profile first. Verified numbers and branded call both require an approved business profile. Enter your registration number exactly as it appears on your documents. Use your current address and a physical phone line as the contact number, since VoIP numbers are not accepted. Make sure your website reflects your business name.
• Verified phone number. Retell registers the number with carriers so it is not marked as spam. Allow 1 to 2 weeks for approval. It costs $10 per number per month.
• Branded call. Recipients see your business name instead of a number. Allow 1 to 2 weeks. It costs $0.10 per outbound call. Prepare a short name and a long name, because Verizon displays up to 15 characters and T-Mobile and AT&T up to 32. Retell recommends pairing branded call with a verified number, because a branded call can still be marked Spam Likely.
• Warm-up. Start each new number on low volume and add traffic gradually.
• Monitoring. Watch for calls ending in marked_as_spam, network_blocked or SIP error 608.
• Call screening. iPhones on iOS 26 can ask unknown callers for their name and reason for calling. Write the agent prompt so it answers that question in one sentence.

STIR/SHAKEN is the industry framework in which the originating carrier digitally signs a call and attests whether the caller is authorized to use that number. Retell's docs do not describe attestation levels. Ask your carrier for imported numbers, or Retell for managed numbers, what attestation your outbound calls receive.

What should caller ID show, and what happens when people call back?

Pick one number, or a small fixed set, per campaign and keep it stable. Each new number needs its own warm-up, so rotating through fresh numbers restarts that work. Every outbound call must come from a number in your workspace, either bought or imported.

Decide the callback path before launch, because some recipients will call back a number they missed. You can bind an inbound agent to the outbound number so callbacks get answered. You can also leave the inbound agent unset if you do not want callbacks, but then a missed call has no path back to you. Keep the branded name, the agent's greeting and your business profile name the same.

On a bring-your-own trunk, cold transfers using SIP REFER only work if your carrier allows SIP REFER and PSTN transfer. Confirm with your carrier that the transferred call shows the caller ID you expect.

Do you need A2P 10DLC registration?

Only if the agent will send texts. Voice calls do not need A2P registration. If the agent texts from your own number, the number needs an approved business profile, a brand and a campaign. That takes 2 to 3 weeks, sometimes longer. The fees are a $4 or $45 one-time brand fee, a $15 campaign fee, and then $20 per number per month plus $0.01 per message. SMS works on Retell Twilio numbers and on imported numbers that have passed A2P. It does not work on Telnyx numbers, toll-free numbers or non-US numbers.

If the agent only needs to send a confirmation during the call, it can send from Retell's pool of SMS-approved numbers with no A2P application. The message is a preset Retell template you cannot customize.

What does IT need to line up before the first call?

On a Retell-managed number, IT has no telephony infrastructure to set up, only calls per second and concurrency. On a bring-your-own number, give your telecom team this list:

• Allowlist Retell's SIP signaling and media IP ranges. The custom telephony docs list the current blocks.
• Open RTP/RTCP on UDP ports 10000 to 65535.
• Offer the PCMU, PCMA or G.722 codec.
• Choose a transport: TCP (recommended), UDP, TLS or mTLS. SRTP media encryption requires TLS, and you set up mTLS through Retell support.
• Provide the trunk's termination URI and, if you use them, the username and password.
• Enable SIP REFER on the trunk if the agent will cold-transfer calls.
• Format destination numbers in E.164.
• Set calls per second within your trunk's capacity. The default is 1.
• Size concurrency. Pay-as-you-go includes 20 concurrent calls, and each extra concurrent call is $8 per month. Reserve inbound concurrency so an outbound campaign cannot crowd out inbound calls.
• Run Test SIP Connection, then a real outbound and inbound call. The connection test alone does not prove routing or audio.

What is the full launch checklist?

For the call itself, turn on Do-not-contact Handling on the agent so opt-out requests mark the contact. Set honor_internal_dnc on outbound calls so marked contacts are skipped. Both are off by default. Consent rules are covered in the TCPA compliance guide for outbound AI voice calls.

Get the number ready before the agent

The business profile gates verified numbers, branded call and A2P, so submit it the day you decide to test outbound. Then apply for verified phone numbers and branded caller ID on the US numbers you will call from, so the first campaign does not start at Spam Likely.

Related reading: The crm-to-voice ai workflow, the call closure load test, and more guides in the Retell Learn hub.