Guide
AI Phone Agents and Your Phone System: Latency, Transfers, SIP
The demo sounds great on a laptop. Then it goes on a real phone line, and callers talk over it, transfers drop and caller ID says "unknown." Here's what telecom people check before an AI agent answers a single customer call.
Most AI phone agent projects don't fail on the AI. They fail on the phone part: the audio path, the transfer, the caller ID, the menu the business already had. Those are old, unglamorous problems, and they're the ones we've spent our careers on.
This guide covers what matters when you connect an AI voice agent to a real phone system: where latency comes from and why it decides whether callers stay on the line, the four common ways to wire an agent in (including SIP integration), how to transfer a call to a person without making the caller start over, and a test checklist to run before launch. If you'd like help with any of it, that's what our AI phone agents work covers.
Latency is the whole game
People are extremely sensitive to pauses on a phone call. A reply that starts promptly sounds attentive. A gap of a second or more sounds like the line dropped, or like a robot thinking, and callers start talking again just as the agent begins to answer. Now both parties are talking over each other and the conversation falls apart.
Voice AI latency isn't one number. It's a chain, and every link adds delay:
| Stage | What adds delay | What to ask the vendor |
|---|---|---|
| End-of-turn detection | Deciding the caller has finished speaking. Too eager and it interrupts; too cautious and it leaves dead air. | Can we tune end-of-turn sensitivity per use case? |
| Speech-to-text | Transcribing the caller, especially on noisy cell connections. | Is transcription streaming, and how does it handle phone-quality audio? |
| The model | Generating a reply, plus any lookups it makes (account status, calendar, ticket). | Which lookups happen during the call, and how are slow ones handled? |
| Text-to-speech | Producing the first words of audio. | Does audio start streaming before the full sentence is generated? |
| Network and media path | Extra hops between carriers, regions and providers, transcoding and jitter buffers. | Where is audio processed relative to our callers and our phone system? |
Two other things decide whether a call feels natural:
- Barge-in. Callers interrupt. When they do, the agent has to stop talking immediately and listen. That needs good echo cancellation, or the agent hears its own voice through the caller's speakerphone and interrupts itself.
- Covering slow lookups honestly. A short "let me pull up your account" while a lookup runs is fine, because a person would say it too. Filling every pause with chatter isn't.
Four ways to connect an AI agent to your phones
There's no single right answer. The best option depends on what phone system you have, who owns your numbers, and how much control you need over transfers.
Call forwarding
The quickest start: forward unanswered or after-hours calls to a number the AI provider gives you. No changes to your phone system. The catch is that forwarded calls can lose context. Depending on the carriers involved, the agent may see your number instead of the caller's, and transferring the caller back to your team usually means a second call out through the public network, which costs another call leg and adds another place for things to break.
A SIP trunk to the agent platform
Your phone system, or the session border controller in front of it, routes calls to the agent over SIP. This is the most control you can get. You can pass context in SIP headers (who's calling, which number they dialed, which queue they came from, the account number if your IVR already collected it) and hand calls back through SIP transfer methods without leaving your own system. It's also the most setup: codec agreement, encryption (TLS and SRTP), firewall and NAT rules, and testing that transfers actually work with your specific platform.
The agent as an extension or queue member
Many hosted PBXs let you register the agent like a phone, or add it to a queue or hunt group. That makes it easy to use as overflow: if nobody picks up after a few rings, the agent answers. Transfers stay inside your PBX, which is usually the most reliable path back to a person.
A dedicated number pointed at the agent
You can route a specific phone number (an after-hours line, a campaign number) straight to the agent provider at the carrier level. It's clean for a dedicated use, but now two systems own pieces of your phone presence. Be clear about who controls the number, and what happens if you ever leave the provider.
| Option | Setup | Caller context | Transfers back | Best for |
|---|---|---|---|---|
| Call forwarding | Easy | Sometimes lost | Out and back over the public network | A quick after-hours pilot |
| SIP trunk | Most work | Rich, via SIP headers | Inside your system, if supported | Main lines, real integration |
| Extension or queue member | Moderate | Whatever your PBX passes | Inside your PBX | Overflow and busy periods |
| Dedicated number | Easy | Good for that number | Depends on the provider | A separate line or campaign |
AI call transfers: where most deployments fail
The moment a caller asks for a person is the moment your AI phone agent gets judged. A cold transfer, where the caller lands in a queue and the human picks up with "how can I help you?", throws away everything the caller just explained. People forgive talking to a machine. They don't forgive repeating themselves.
A warm transfer carries context with the call. There are a few ways to do that:
- Context in SIP headers. The agent attaches the caller's name, account and a one-line reason to the transfer, and your phone system or contact center shows it to the person who answers.
- A screen pop. The agent writes a short summary to the ticket or CRM record before the transfer connects, and the human's screen opens on it.
- A spoken handoff. The agent briefs the human privately, then connects the caller: closest to how a good receptionist works, but slower and harder to set up.
The failures we see most often aren't exotic:
- Transferring to a queue that's closed, so an after-hours caller who asked for a person reaches a dead end. After hours, the honest answer is a clear callback promise, not a transfer.
- Transfer loops: the human queue overflows back to the AI, which transfers back to the queue.
- A transfer method one leg of the call doesn't support, so the call simply drops.
- Silence on hold, so callers assume they've been cut off and hang up.
Our rule of thumb: if someone asks for a person twice, they get a person, or a callback with a specific time. No third attempt at persuasion.
IVRs, DTMF and the menu you already have
Plenty of callers still press zero the moment a machine answers. Respect it: zero should still reach a human or a callback offer. Make sure keypad presses (DTMF) survive the whole audio path. Tones sent as proper telephony events usually do; tones sent as plain audio can be garbled by transcoding along the way, and then "press 1 for billing" quietly stops working.
If you already have an IVR menu, decide on purpose where the AI sits. It can replace the menu, sit behind one option ("press 2 to talk to our assistant"), or handle overflow when the queue is full. What you shouldn't do is make callers go through both a menu and a chatty agent before reaching anyone.
Audio quality and codecs
Most calls that cross the public phone network are narrowband audio, which is far lower fidelity than the recordings speech models are usually demoed on. Your agent has to understand phone-quality audio, accents, background noise and people in cars. Wideband codecs help only when the whole path supports them end to end, and every unnecessary transcoding step makes things worse. Packet loss and jitter turn into garbled transcripts, and garbled transcripts turn into confidently wrong answers. Test from bad cell connections, not just your office.
Caller ID, outbound calls and the rules
Inbound: make sure the agent sees the caller's real number, especially if calls are forwarded. Use it to look the caller up, but verify identity before discussing account details. A caller ID match isn't proof of who's calling.
Outbound: carriers now label calls based on caller ID authentication. Under the STIR/SHAKEN framework, calls are signed with an attestation level, and calls from numbers your provider can't vouch for are far more likely to show up as "Spam Likely." Make sure outbound calls go out from numbers you own, signed by a provider that can attest to them. The FCC's call authentication page explains the framework.
Consent matters even more. The FCC has ruled that AI-generated voices count as "artificial" voices under the TCPA, so automated outbound AI calls need the called party's prior express consent. Our own rule is stricter than the minimum: the AI only calls people who asked to be called, it says up front that it's automated, and it always offers a person.
A pre-launch test checklist
Run these on real lines, from real phones, before a single customer hears the agent:
| Test | How | Passes when |
|---|---|---|
| Latency on a real line | Call from a cell phone and ask a multi-part question | Replies start promptly, with no dead air |
| Barge-in | Interrupt the agent mid-sentence | It stops at once and answers the new question |
| Speakerphone and car | Call on speaker in a noisy room and from a moving car | No echo loops; transcripts stay accurate |
| Warm transfer, business hours | Ask for a person | The human sees a summary; the caller never repeats themselves |
| Transfer after hours | Ask for a person at night | A clear callback promise, never a dead queue |
| Keypad | Press zero and the menu digits | Every digit is recognized; zero reaches a person or a callback |
| Caller ID | Call from a known customer number, forwarded and direct | The agent sees the real number and still verifies identity |
| Failure | Take the agent offline mid-shift | Calls fall back to a person or voicemail, not a busy signal |
| Outbound labeling | Place test calls to phones on each major carrier | No spam label; the right name where supported |
| Recording and disclosure | Listen to the first seconds of a call | Required notices play, including that the caller is talking to an automated assistant |
Start with overflow, not the front door
The safest first deployment is the call you're already missing: after hours, lunch, or overflow when everyone's busy. Callers there would otherwise reach voicemail, so the bar is low and the upside is real. Measure abandoned calls, transfers and repeat callers against what you had before. Move the agent closer to the front door only when those numbers hold up.
If you want help picking the right integration path for your phone system, or a second opinion on a vendor's demo, that's a good use of a free 30-minute call.