You've spent years building your phone system. Extensions are assigned, hunt groups route calls to the right people, and your IVR menus do their job. Maybe it's a 3CX deployment, a FreeSWITCH box, or something your telco set up a decade ago. The good news: you don't need to rip any of it out to connect a legacy PBX to an AI voice agent. You just need a bridge.
Your PBX still works — it just needs a new extension
The core idea is simple. A SIP connector lets your existing PBX hand calls off to an AI agent, the same way it would hand a call to any other extension or external destination. Your PBX stays in control of routing. The AI agent slots in as another endpoint — one that happens to answer, transcribe, and action calls automatically.
From your PBX's perspective, the AI agent is just another SIP device. It registers like a handset or softphone would. There's no need to change your trunk provider, re-number your extensions, or rewrite your dial plan from scratch. The AI sits alongside everything you already have.
Three common setups that work today
Most businesses connecting a legacy PBX to an AI voice agent use one of three patterns. Which one suits you depends on when and how you want the AI to pick up.
- Overflow — AI answers when no one picks up. Your PBX already has a no-answer timeout — usually 15 to 30 seconds. Instead of sending unanswered calls to voicemail, you point that overflow destination at the AI agent. Your team gets first crack at every call. The AI only steps in when nobody is available. This is the most common starting point because it changes almost nothing about your existing call flow.
- After-hours — AI handles calls on a time-of-day rule. Most PBX platforms support time-based routing. Between 8am and 5pm, calls go to your team as normal. Outside those hours, calls route to the AI agent. The caller gets a real conversation instead of a voicemail greeting, and you get a summary and urgency score waiting for you in the morning.
- Dedicated DID — AI sits on its own number for specific enquiry types. If you want the AI to handle a particular category of calls — say, general enquiries while your team focuses on existing clients — you assign a dedicated DID to the AI agent. You can advertise that number on your website or Google Business listing, and all calls to it go straight to the AI. Your main number and extensions stay untouched.
None of these require you to migrate away from your PBX. They're additive — you're bolting on a new capability, not replacing what you already have.
What stays the same
This is the part that matters most when you've invested in a working phone system. When you connect an AI voice agent via SIP, your existing infrastructure keeps doing its job:
- Extensions and ring groups — your team's handsets and softphones don't change
- Hunt groups and queues — calls still distribute the way you've configured them
- IVR menus — callers who press 1 for sales and 2 for support still land in the right place
- Call transfer rules — the AI can transfer calls back to a live person on your PBX if needed
- Recording and CDR — your PBX still logs everything on its side; the AI adds its own transcription and summary on top
The AI doesn't take over your phone system. It fills the gaps — the calls nobody could get to, the hours nobody was available, the enquiries that would have otherwise gone to voicemail and never been returned.
How the SIP connection works
At a technical level, the AI agent registers with your PBX as a SIP endpoint. You configure a SIP trunk or extension on your PBX that points to the AI service. When your PBX forwards a call — whether on overflow, after-hours, or via a dedicated DID — the SIP session is established directly between your PBX and the AI platform.
The AI agent answers, has a natural conversation with the caller, captures the details, and then sends you a call summary with urgency scoring. If the caller needs to speak to someone immediately, the AI can transfer the call back to your PBX — to a specific extension or ring group — using a standard SIP REFER or re-INVITE.
This works with any PBX that supports standard SIP trunking: 3CX, FreeSWITCH, Asterisk, Yeastar, Sangoma, or even hosted platforms. If your PBX can send calls to an external SIP URI, it can send calls to an AI agent.
De-risking the decision
Adding an AI voice agent to your PBX is low-risk for a few reasons:
- It's reversible — if you don't like it, you point your overflow or after-hours destination back to voicemail. Nothing else changes.
- It's incremental — start with after-hours only. See how it performs. Then expand to overflow during business hours if it works for you.
- It doesn't touch your trunk — your SIP trunk provider, phone numbers, and porting arrangements stay exactly as they are.
- It doesn't require new hardware — the AI agent is a cloud service. There's nothing to install on-site.
For businesses that have spent real money on their PBX — and real time configuring it — this matters. You're not being asked to start from zero. You're being asked to plug in one more endpoint.
Getting started with dareena.ai
dareena.ai works with standard SIP trunks and devices, making it straightforward to connect to your existing PBX. The AI agent registers as a SIP endpoint, answers calls with a natural NZ voice, and sends you transcribed summaries with urgency scoring. It connects to tools like Google Calendar, Trello, Google Sheets, and Telegram so call data flows into the systems you already use.
Setup is conversational — you don't need to read documentation or configure a complex admin panel. And with a 7-day free trial (no credit card required), you can test it against your real call flow before committing. Plans start from $50/month plus GST.
Your PBX isn't the problem. The gap is what happens when nobody picks up. An AI voice agent fills that gap — without asking you to change anything that already works.