This guide shows how to connect a Twilio SIP trunk to AI voice agents in VoiceOI: create an Elastic SIP Trunk in your own Twilio account, add it to the Carrier Manager, test it and attach it to an outbound campaign. Twilio carries the calls and bills the minutes; VoiceOI runs the bots and bills per bot.
VoiceOI uses your Twilio trunk the same way it uses any SIP carrier. Twilio connects the call to the phone network; VoiceOI handles everything that happens on the call.
Twilio and Elastic SIP Trunking are products of Twilio Inc. You create and pay for the trunk in your own Twilio account.
Four steps in the Twilio Console, then four in VoiceOI. The Twilio steps follow Twilio's own documentation as reviewed in October 2026; if a console label has moved, Twilio's docs are the authority. The steps are described in text and no console screenshots are shown, because Twilio changes its labels often.
In Twilio
In the Twilio Console, go to Communications, then Elastic SIP Trunking. Open Trunks and click Create New Trunk, then give it a friendly name you'll recognize, such as “VoiceOI outbound”.
In Twilio
Under Termination, choose a SIP domain name unique to this trunk, in the form yourname.pstn.twilio.com. That is the address your calls are sent to on their way to the phone network.
In Twilio
Twilio requires at least one method: a credential list (username and password) or an IP access control list. If you set both, Twilio enforces both. Twilio recommends credentials, because an IP list alone doesn't protect against some attacks. Keep the username and password for step 5. This page doesn't list VoiceOI's sending IP addresses, so if you need an IP access control list, ask sales for them first.
In Twilio
Twilio accepts only a caller ID that is a Twilio number on your account or a number verified in the Console, so buy a Twilio number or verify the one you'll show. Trial accounts can call only verified numbers. Origination settings are for receiving inbound calls, so this outbound setup doesn't need them.
In VoiceOI
Add a carrier and paste a sip.conf or pjsip.conf block built from these Twilio values, then save it:
In VoiceOI
Build the script in the no-code voice bot builder, as a recorded-voice bot or an AI agent, and run it in the simulator before it calls anyone.
In VoiceOI
Upload a one-row CSV with your own phone number, set up the campaign to dial through the Twilio carrier and start it. Check that the call rings, shows your caller ID and appears in VoiceOI's call log, then review its recording and transcript. If it fails, the troubleshooting answers below cover the errors Twilio documents for outbound trunk calls.
In VoiceOI
Load your lead list, dial through the Twilio carrier and set pacing so new calls don't start faster than your trunk's calls-per-second limit; Twilio starts each trunk at 1 call per second per region. Add calling windows and retries, then launch.
Twilio steps from Twilio's Elastic SIP Trunking docs and calls-per-second guide, reviewed October 2026.
Twilio isn't required. Any SIP carrier is added the same way, as a block in the Carrier Manager; see the Telnyx SIP trunk for AI voice bots guide or the general SIP trunk for AI voice agents page, and use the SIP trunk selection guide to compare carriers on the same seven checks. If your floor already dials on VICIdial, the bots can join it as remote agents instead; see the VICIdial AI integration.
Twilio's job is carrying the call. What the person hears, and what your team gets afterwards, comes from the VoiceOI bot.
More on AI live transfer and answering machine detection for call centers.
A Twilio trunk doesn't change the consent rules. Since the FCC's February 2024 ruling, AI voices count as “artificial” under the TCPA (FCC news release), so telemarketing calls need prior express written consent, and your organization stays responsible. This isn't legal advice. See TCPA controls for AI voice bots and do-not-call (DNC) controls.
Every Twilio setup has two cost layers: Twilio's charges for minutes and numbers, and whatever runs the AI. Twilio's US price list shows termination to the 48 states (Zone 1) at $0.0100 a minute and local numbers at $1.15 a month, with unlimited call concurrency included. Those Twilio charges are the same whichever AI layer sits on top.
Per-minute voice-AI platforms add a charge to every connected minute. Vapi lists $0.05 a minute for hosting, with transcription, LLM and voice models passed through at cost on top. Retell lists $0.07 to $0.31 a minute for AI voice agents and doesn't charge for SIP trunking or custom telephony. VoiceOI bills per bot per month instead, with no per-minute AI fee. Bots are priced per bot per month by tier: Hosted SIP tiers list from $45 to $155 and VICIdial / Asterisk tiers from $25 to $135, before volume discounts of up to 60%; the pricing page shows the tier that fits your setup.
Concurrency is priced differently too. Vapi lists 4 concurrent calls without a paid Success Package and $10 per extra line per month; Retell lists its first 20 concurrent calls free, then $8 per concurrent call per month. On VoiceOI, you pay per bot (seat), and volume discounts count concurrent bots.
| Cost line | 1,000 connected minutes | 5,000 connected minutes |
|---|---|---|
| Twilio termination, 48 states (Zone 1), in either setup | $10 | $50 |
| Vapi hosting fee, before model costs | $50 plus models | $250 plus models |
| Retell AI voice agents, stated range | $70 to $310 | $350 to $1,550 |
| VoiceOI, SIP Simple Fronter list price (the lowest Hosted SIP tier; other tiers cost more) | $45 per bot, flat | $45 per bot, flat |
The example assumes every minute bills at the listed rate. Real bills depend on destinations, rounding, model choice, add-ons and discounts. VoiceOI's tiers differ in what they include (question limits, closer notes, live transfers), so compare the tier you would actually use. Speech recognition comes in two tiers: the Standard Voice Engine, included at no extra cost, and the Advanced Voice Engine for longer conversations, listed at 2 Workers Free (then $15/worker).
Rates from Twilio's US Elastic SIP Trunking pricing (the page says it is current as of August 2026), Vapi's pricing page and Retell AI's pricing page, all reviewed October 2026. Run your own numbers with the AI calling cost per minute guide, or see the voice AI cost comparison and AI voice bot pricing.
Add the trunk in the Carrier Manager, build a bot without code and place a test call to your own phone first.
The errors Twilio documents for outbound trunk calls, what they mean, and how billing and scope work.
Twilio returns 403 Forbidden when a call arrives from an IP address that is not on the trunk's IP access control list, or when the username or password does not match its credential list. Check that the credentials in your carrier block match the credential list exactly. If you use an IP access control list instead, make sure the address the call comes from is on it.
Twilio needs every dialed number in E.164 format with the plus sign, for example +12125550123 for a US number, rather than 10 or 11 digits on their own. Check how numbers are formatted in your carrier settings.
The caller ID on the call is not a Twilio number on your account or a verified caller ID. On a trial account, both the number you call and the caller ID have to be verified.
New calls are starting faster than the trunk's calls-per-second (CPS) setting allows; by default that is 1 call per second per trunk per region. You can set a trunk's CPS between 1 and 5 in the Twilio Console. Above 5, Twilio's sales team handles it, and extra CPS per region carries a monthly fee. Until then, lower the pacing on your campaign.
Twilio limits trial accounts to 4 concurrent calls and returns 503 Trunk concurrent call limit exceeded beyond that; upgraded accounts without an approved Business Profile can hit the same error. Upgrade the account and get the Business Profile approved, or run fewer bots at once until then.
The destination country isn't enabled in your Twilio account's Voice Geographic Permissions. Enable the countries you actually call, and only those.
Twilio uses 603 when the account is not active, for example closed or suspended for lack of funds or for breaking its Terms of Service or Acceptable Use Policy. Check your balance and account status in the Twilio Console.
No. VoiceOI charges a prepaid price per bot per month with no per-minute AI fee. Bots are priced per bot per month by tier: Hosted SIP tiers list from $45 to $155 and VICIdial / Asterisk tiers from $25 to $135; the pricing page shows which applies to your setup. Twilio bills your account for minutes and numbers.
No. VoiceOI uses the trunk for outbound calls: bots dial your leads and transfer qualified ones to your closers. It isn't an inbound receptionist or a call-routing IVR, so you don't need Twilio's Origination settings.
Yes. Telnyx and other SIP carriers are added the same way, as a sip.conf or pjsip.conf block in the Carrier Manager.
Error codes and causes from Twilio's SIP trunking troubleshooting guide and calls-per-second guide, reviewed October 2026.
Third-party details on this page come from these pages, reviewed October 2026. Prices, limits and console labels change often, so check the source before you rely on them.