How to Add an AI Voice Agent to Your Existing Phone Number

There are three ways to add an AI voice agent to your existing phone number. You can forward calls from your number to one the agent answers. You can port (transfer) your number to a carrier that connects to the agent. Or you can connect your current phone system to the agent over a SIP trunk. Forwarding is the quickest to set up and the easiest to undo, while porting and SIP give you more control in exchange for more setup work.

Whichever route you choose, your customers keep dialling the same number. What changes is where the call is answered, who controls the routing, and how easily you can switch back if something goes wrong. This guide covers each option, what it needs from you, and what to check before you go live.

First decide which calls the agent should answer

Settle this before you change any telephony settings, because it affects which option suits you. Most businesses pick one of these patterns:

All calls: the agent answers every call, then deals with it or passes it to a person.
After hours only: staff answer during the working day and the agent covers evenings, weekends and holidays.
Overflow: the agent picks up when staff are busy or a call rings out.
One line or department: for example, the bookings line goes to the agent and the accounts line stays with people.

The more conditions you add, the more you need routing logic you control, rather than a simple forwarding setting.

Option 1: Forward calls to the agent's number

With forwarding, your number stays with your current provider. The voice platform gives the agent a new number, and calls to your main number are forwarded to it. A guide from one AI receptionist vendor shows how this works with Twilio. You set up a forwarding rule in Twilio, using a TwiML Bin or a webhook, so that calls go to the agent. The guide describes three common set-ups: always forward, forward only after hours, and forward only when your team has not picked up within a few rings.

How it works in practice

Build and test the agent on its own new number.
Choose the type of forwarding you want (every call, unanswered calls, busy calls or by schedule) and set it up in your current provider's portal or on the voice platform.
Call your main number from a mobile and confirm that the agent answers and the call sounds right.

Trade-offs

Fast and reversible. If the agent misbehaves, you switch forwarding off and you are back where you started.
Charges for forwarded calls. Ask your carrier how it bills forwarded minutes, because the voice platform will charge for the inbound call as well.
Caller ID. If the agent needs the caller's number to look up their account, check with your carrier that the original number comes through on a forwarded call, and test it before launch.
Your carrier's features set the limits. If your provider cannot forward on a schedule, you can forward every call and apply the time-of-day rules on the voice platform instead.
Outbound calls and texts. If the agent also makes calls or sends texts, check which number recipients will see. The agent's own number is not the one your customers know.

Forwarding suits a pilot, a single-line business, or any team that wants proof the agent works before committing.

Option 2: Port your number to a voice-capable carrier

Porting moves the number itself to a new carrier, usually the telephony provider your voice platform uses. Calls then arrive directly at the infrastructure that runs the agent, with no forwarding step in between.

Your right to keep the number

In the US, the FCC's consumer guidance says you can usually keep your number when you switch providers, as long as you stay in the same geographic area. This applies between landline, wireless and internet (IP) providers. In the UK, Ofcom's rules give you the right to keep your number when you switch provider and require the port to be completed within one working day. In practice, ask both the old and new provider for their expected timescale for your type of number.

What you will need

Twilio's US porting guidelines are a typical example of what carriers ask for:

A signed Letter of Authorisation (LOA).
The account holder's name and address exactly as they appear on the Customer Service Record (CSR) held by your current provider.
A copy of a phone bill dated within the last 30 days.
Your account number with the current provider, plus a PIN if it uses one.

Twilio says incorrect name or address details are the most common reason a port is rejected. Ask your current provider for the exact details on file before you submit anything.

How long it takes

Twilio puts standard US ports at 5 to 15 days once all documents are submitted. If the current provider rejects the request it can take four weeks or longer, and bulk ports of 50 or more numbers take 4 to 6 weeks. Twilio does not guarantee a specific completion date, so do not plan a launch around a fixed day.

What to watch for

Keep your old service running. Twilio advises keeping your existing service until the port completes, to avoid an interruption. Close the old account only after the number has moved.
Know what else uses the number. Desk phones, fax lines, alarm lines, card terminals and text messaging may all depend on it. Once the number moves, each of them needs a new arrangement.
Agree a fallback before cutover. Set the new carrier to send calls to a staff mobile or to voicemail if the agent is unavailable. Configure this before the port date, not after.
Think about lock-in. If the number sits inside a voice-agent product's own account, find out how you would port it out again if you change vendors.

Option 3: Connect your existing phone system over a SIP trunk

If you run a PBX or a cloud phone system with extensions, queues or several sites, you may not want to move the number or rely on forwarding. A SIP trunk lets your carrier or phone system send calls to the voice platform over the internet while the number stays where it is. Vapi, for example, calls this a "bring your own SIP trunk" connection.

What setup involves

Vapi's documentation shows what is typically involved:

Collect the SIP server address, authentication credentials and at least one phone number from your provider.
Create a SIP trunk credential on the voice platform and link your number to it.
Add the platform's IP addresses to your provider's allowlist. Vapi also lists a UDP port range (40000 to 60000) for call audio.
Test both outbound and inbound calls before routing live traffic.

Compatibility pitfalls

Authentication method. Vapi generally advises against IP-based authentication for SIP trunks. Its servers are shared, so calls can be misrouted unless your provider gives each customer a unique endpoint.
Data region. Vapi's EU region is available only with its Premier Success Package, and EU-hosted credentials must be created through its EU API endpoint. If you need call audio processed in the EU, confirm where every component runs: telephony, speech-to-text, the language model and text-to-speech.
Your phone system's capabilities. Before committing, check that your PBX or provider supports the connection method the voice platform expects.

SIP is the most flexible option. It is also the one where your phone system provider and your voice-agent builder most need to work together.

Plan how calls get back to people

Connecting the number is only half the job. The agent also needs a reliable way to hand callers to staff. Decide which calls go to which person or team, what happens when nobody answers, and whether the agent passes a summary to whoever takes the call. With forwarding or porting, transfers usually go to staff mobiles or direct numbers. With SIP, the agent may be able to transfer back into your PBX extensions or queues, depending on your system.

In a voice receptionist we built, the agent answers every call, day or night, on the client's existing phone number, then classifies each call and routes it. A managed voice platform decides a great deal about how calls behave, so we ran classification and routing on our own server. That keeps the routing logic under our control and lets us change it without waiting on the platform. We also ran speech-to-text and text-to-speech through a single provider. If you expect your routing rules to change, ask where that logic will live before you sign.

Tell callers they are speaking to AI

Article 50 of the EU AI Act applies from 2 August 2026. It requires that people interacting directly with an AI system be told so, unless that is already obvious. The European Commission's guidance says the notice should come at the start of the first interaction and be clear and distinguishable. A voice agent that holds a conversation with callers falls within this, and a short line in the greeting is a straightforward way to comply. You should also check the local rules on call recording and consent in each country you serve.

A cutover checklist

List every service that depends on the number today.
Choose forwarding, porting or SIP, and confirm your carrier supports it.
Build the agent and test it on a temporary number with realistic scenarios, including angry callers, poor lines and callers asking for a person.
Set up a fallback route to a person or voicemail.
Check that caller ID, transfers and any CRM lookups work end to end.
Go live outside peak hours and keep someone on call.
Review recordings or transcripts from the first week and adjust the routing.

Questions to ask a voice-agent provider

Which connection methods do you support: forwarding, porting, SIP, or all three?
If we port the number to you, who owns it, and how would we move it out again?
Where does the routing logic run, and can we change it ourselves?
What happens to calls if your platform or the AI model is down?
In which regions are call audio and transcripts processed and stored?

Khive builds AI voice agents in code and hands them over, so the client owns the system. You can read how we built the AI voice receptionist. If you would like an agent answering calls on the number your customers already use, send us a brief describing your phone setup and the calls you want handled.