What is SIP trunking?

A SIP trunk is a voice connection between two systems over IP using:

SIP signaling (call setup, call routing, call teardown)
RTP media (audio stream)

SIP trunking is commonly used to connect:

Call center platforms
VoIP / PBX systems (Asterisk, FreePBX, Cisco, Avaya, etc.)
SIP carriers
External voice services (AI agents, conferencing systems, etc.)

What SIP trunking enables with Callr

With Callr, SIP trunking is useful when you want to bridge your external telephony system with:

Callr phone numbers (DIDs)
Callr call flows (Callr Actions)
Callr APIs
Callr AI capabilities (transcription, call insights, data extraction, etc.)

Typical workflows include:

✅ Receive calls into Callr from an external system
✅ Send calls from Callr to your SIP endpoints
✅ Route calls between call flows and call centers
✅ Implement AI agent workflows with human handoff
✅ Centralize call tracking and analytics

Common use cases

1) Connect Callr to your call center

If you already run a call center platform or PBX and want Callr for routing, tracking, recording, transcription, or AI analysis, SIP trunking makes that integration possible.

Examples:

External call center receives calls from Callr numbers
Callr call flows pre-qualify calls before sending them to agents
Callr collects call metadata and pushes it to your CRM

2) Bring your own telephony infrastructure (PBX / SBC)

SIP trunking lets you connect your existing infrastructure and keep your preferred setup while using Callr for higher-level call logic and services.

Examples:

Use Callr as your programmable voice layer
Use your own SBC for NAT/security and route calls to/from Callr
Keep internal extensions and call routing rules in your PBX

3) Connect Callr to external systems

Some systems expose SIP endpoints that can place or receive calls (voice bots, conferencing platforms, telecom gateways, etc.). SIP trunking lets Callr communicate with those systems.

Examples:

Dial external SIP URIs
Route calls to multiple SIP targets
Perform transfers to external systems

4) Build voice agent experiences (OpenAI, ElevenLabs, etc.)

Modern voice agents often require:

real-time audio streaming
programmable call control (hangup, transfer, gather digits, etc.)
structured call data (transcripts + insights)

SIP trunking is a natural fit to connect Callr call handling with:

AI voice agents (real-time conversational)
Text-to-Speech (TTS) providers
Speech-to-Text (STT) providers

A typical pattern is:

Call enters Callr (inbound DID or outbound call)
Callr routes audio to your AI agent system
AI agent speaks and listens in real time
Call can be transferred to a human agent when needed

Inbound vs outbound SIP

Inbound SIP (External -> Callr)

Inbound SIP means an external system sends calls to Callr, for example:

A PBX routes calls to Callr
A carrier forwards traffic to Callr
A call center platform sends calls into Callr

Use inbound SIP when Callr needs to:

handle calls with Callr Actions
apply call logic before routing elsewhere
track calls and push data to your platform

Outbound SIP (Callr -> External)

Outbound SIP means Callr sends calls to an external SIP endpoint, for example:

Callr calls a SIP-enabled call center queue
Callr calls your PBX / SBC
Callr routes calls to an AI voice agent running behind SIP

Use outbound SIP when Callr needs to:

dial external SIP URIs
bridge calls between PSTN and your infrastructure
connect to third-party systems

How SIP trunking fits in the Callr ecosystem

SIP trunking is one way to connect systems to Callr, alongside:

PSTN calls to/from Callr numbers (DIDs)
Callr Actions call flows (programmable call logic)
REST APIs (call initiation, tracking, reporting)
Data integrations (OAuth, push/fetch data from your platform)
AI features (transcription, call summaries, extraction)

You can combine SIP trunking with the rest of the platform to build advanced voice workflows, such as:

inbound call routing based on customer data
AI-driven call qualification
call tracking and conversion analytics
call transfers and fallback scenarios
automatic CRM enrichment after each call

When you should use SIP trunking

You should use SIP trunking if:

You already have SIP infrastructure (PBX/SBC/call center)
You need enterprise-grade voice interoperability
You want to connect Callr to external voice agents
You want more control than basic PSTN routing

If you only need simple outbound dialing and call tracking from phone numbers, you may not need SIP trunking at all.

Next steps

If you want to integrate SIP trunking with Callr, continue with:

Getting Started with SIP Trunking
SIP Inbound Connectivity
SIP Outbound Connectivity