Your business already has a phone system, a team that answers calls and a set of numbers that customers know. Adding an AI voice agent should not mean rebuilding all of that. The practical question is how calls reach the agent, and how they come back to your staff. A SIP trunk AI voice agent setup is one of the main answers, though not the only one.
An AI voice agent can take your calls in three ways: a new local number, forwarding from your existing number, or a SIP trunk to your phone system. SIP is the protocol phone systems use to set up and end calls. The right route depends on whether you keep your numbers.
What is SIP, in plain words?
SIP stands for Session Initiation Protocol. It is the signalling standard that phone systems and internet telephony use to set up, change and end calls. The IETF standard describes it as "an agile, general-purpose tool for creating, modifying, and terminating sessions" (RFC 3261). SIP carries the instructions for a call. The voice itself travels separately, as audio. For where telephony sits within a whole call, see how AI voice agents work.
Two terms help when you talk to your telecoms team. A PBX (private branch exchange) is the phone system inside your business that routes calls to extensions, queues and staff. A SIP trunk is a connection between that system and a telecoms provider, carrying many calls at once over the internet.
Which option fits your phone system?
Each route trades simplicity against control. The table sets out the main differences.
| Option | What changes | Keep your existing number? | Good when | Watch out for |
|---|---|---|---|---|
| New local number | The agent gets its own number, which you publish | Not needed, but customers must learn the new number | You are starting fresh, or the agent handles a new line | Customers keep dialling the old number unless you redirect it |
| Call forwarding | Calls to your existing number are sent on to the agent | Yes | You want a quick start without changing your phone system | Less control over which calls go where, so test busy and no-answer cases |
| SIP trunk to your PBX | Your phone system sends selected calls to the agent over the trunk | Yes | You want to choose which calls go to the agent and bring them back | Needs your telecoms team or provider, and testing of every transfer |
Many businesses start with forwarding for a short trial, then move to a SIP trunk once they want control over routing. Either way, test the full path from the caller to the agent and back again.
Forwarding or SIP: what is the difference?
Forwarding is the simpler route. Your provider or phone system sends unanswered or selected calls to the agent's number, and your PBX keeps working as it does today. There is little to configure, but you have less say over which calls go where.
A SIP trunk gives you control. Your PBX can decide which calls go to the agent, at what times, and how a call returns to staff. It needs more setup, and it needs a clear owner for the connection.
Callers usually cannot tell which route is in use. Your team will notice the difference in caller ID, in transfers and in reporting. Ask which route your provider supports, and which number the agent will see as the caller's number.
Can you keep your existing numbers?
Often, yes. Both forwarding and SIP let calls to an existing number reach the agent. Moving the number itself to a new provider is a separate process known as porting, and timelines vary by country and by provider. Ask your current provider and the new one for their timelines in writing, and plan for a period when both routes work.
How do calls get back to your staff?
Sometimes the agent should not finish a call. The caller asks for a person, a payment needs a manager, or a complaint needs a human. The agent transfers the call to a staff extension or queue and passes on the summary, so the caller does not start again. The transfer runs through your phone system, so it depends on your PBX rules and on who is available.
Test every transfer path. Try a free extension, a busy extension, an empty queue and a call outside office hours. A transfer that rings into silence is worse than no transfer at all. Warm handoff explains what the summary should contain.
What should you ask your telecoms team?
Bring these questions to your provider before you commit to a route.
- Which calls will reach the agent, and by which route?
- Do we keep our current numbers, and what is the porting timeline?
- How do emergency calls avoid the agent entirely?
- Which codec is used to compress voice on the line, and how does that affect quality? A codec is the method used to pack audio for transmission.
- Which caller ID does the agent see, and what do staff see when a call is transferred?
- Where do calls go if the agent or the connection fails?
- How is call recording disclosed to callers, and does that vary by country?
- Who owns the trunk, who receives its alerts, and who fixes it outside office hours?
How does VoiFlow connect to your phone system?
VoiFlow works with your existing phone system through SIP or a PBX, or with new local numbers, depending on what you already have. Local numbers are available in the UAE and GCC, Malaysia and Southeast Asia, the UK and the US, and calls are hosted in-region. The deployment options describe the set-up steps. Once a call is connected, the integrations cover the systems the agent can read from and write to.
Choosing a route is a decision for your team and your telecoms provider. Start with the table above, and take the questions to your provider before anything is switched on.
Frequently asked questions
Do I need to change my phone system to use an AI voice agent?
Not always. Call forwarding can start without changing your phone system. A SIP trunk needs your PBX or telecoms provider to route calls to the agent. That is a larger change, but it gives you more control.
Can an AI voice agent transfer a call to a person?
Yes, if the route supports it. The agent transfers the call to a staff extension or queue with the summary. Test the transfer for free, busy and out-of-hours cases before launch.
What is the difference between SIP and a SIP trunk?
SIP is the protocol that sets up, changes and ends calls. A SIP trunk is a connection from your phone system to a provider that carries those calls using SIP. SIP is the language, and the trunk is the line that carries it.
Where should I start with my telecoms team?
Start with the question list above, and agree who owns the connection. You can talk to us about your phone system, or try a live call to hear the agent before you connect anything.
