Telephony

SIP Integration with Aircall

SIP trunking is only available on the Synthflow Enterprise Plan.

This guide covers connecting Aircall to Synthflow via SIP trunk so that calls flowing through your Aircall contact center can be routed to a Synthflow AI agent. Synthflow can handle inbound calls, qualify leads, and warm-transfer to live Aircall agents.

Prerequisites

RequirementDetail
AircallActive account with Admin access; SIP/external routing enabled
SynthflowEnterprise Plan with a configured AI assistant (v2.0+)
Phone NumberAt least one DID provisioned in Aircall (E.164 format)
NetworkFirewall allows SIP (UDP/TCP 32681 or TLS 32682) and RTP media between Aircall and Synthflow

Aircall Configuration

All configuration is done in the Aircall Dashboard (dashboard.aircall.io) under Integrations and Smartflows.

Create a SIP Forwarding Destination

Aircall routes calls to external SIP destinations using its Smartflow builder. To route calls to Synthflow:

  1. Go to Integrations → SIP / External SIP.
  2. Click Add SIP Destination.
  3. Configure the destination:
FieldValue
NameSynthflow AI
SIP URIsip.synthflow.ai
Authentication TypeIP-based (whitelist Synthflow IPs)
Port32681 (UDP/TCP) or 32682 (TLS)
TransportUDP/TCP or TLS
  1. Click Save.

Aircall supports IP-based authentication. Add Synthflow’s SIP signaling IPs to the allowlist — contact Synthflow support for the current IP list. If your Aircall plan requires credential-based auth, obtain the SIP username and password from your Synthflow SIP number settings.

Aircall uses SIP INVITE with the DID as the Request-URI. Ensure the Synthflow SIP number matches the DID provisioned in Aircall.

Configure Inbound Routing via Smartflow

Route inbound calls to Synthflow using the Aircall Smartflow builder.

  1. Go to Numbers → select your inbound number.
  2. Click Edit Smartflow.
  3. Add a Forward to External SIP step.
  4. Select the Synthflow AI SIP destination created above.
  5. Set a timeout (recommended: 30 seconds) and configure a fallback destination (e.g., an Aircall team) in case Synthflow does not answer.
  6. Save and publish the Smartflow.

For after-hours or overflow routing, add a Business Hours condition before the Synthflow SIP step to direct calls to AI only when agents are unavailable.

Configure Transfer Back to Aircall (AI → Live Agent)

To enable Synthflow to transfer a call back to a live Aircall agent:

  1. In Aircall Dashboard, note the Line ID, Team ID, or User ID for the destination queue.
  2. These IDs appear in the Aircall dashboard URL when viewing the relevant entity.
  3. Configure Synthflow’s transfer action to route to the Aircall SIP destination with the appropriate queue identifier.

Aircall does not support SIP REFER for mid-call transfers. Synthflow initiates a new outbound SIP INVITE to the agent queue. Consult Synthflow support for the recommended transfer configuration.

Synthflow Configuration

Add the SIP Number

  1. Go to the Synthflow Phone Numbers page.
  2. Click Add Number → Custom / SIP.
  3. Enter the phone number in E.164 format (same DID provisioned in Aircall).
  4. Fill in the SIP connection details:
FieldValue
Termination URIYour Aircall SIP gateway hostname (provided in Aircall SIP settings)
SIP UsernameYour Aircall SIP authentication username
SIP PasswordYour Aircall SIP authentication password
Outbound Proxy (if required)Aircall SIP proxy address and port (if specified by Aircall)

The Termination URI points back to Aircall — this is where Synthflow routes outbound calls and transfers.

Be precise when copying SIP credentials. Incorrect values will cause registration failure.

Attach to the Assistant

  1. Go to Assistants and create or select an assistant (v2.0+).
  2. In the assistant settings, attach the SIP number you just created.
  3. Wait 1–2 minutes for SIP registration to complete.

The SIP trunk is created at the moment of attaching the SIP number to the assistant.

Call Flow Summary

ScenarioFlow
Inbound to AICaller → PSTN → Aircall → Smartflow → SIP Forward → Synthflow AI Agent
AI transfers to live agentSynthflow AI → new SIP INVITE → Aircall → Team/User queue → Live Agent
Overflow / after-hoursAircall Smartflow (Business Hours condition) → Synthflow AI → handles call

Verification

TestWhat to check
Inbound callCall the Aircall DID from an external phone. Verify the Smartflow routes the call to Synthflow AI. Check two-way audio.
Transfer to live agentTrigger a transfer keyword during a Synthflow call. Confirm the call lands in the correct Aircall team with caller ID intact.
Fallback routingAllow the Synthflow step to timeout. Confirm the call falls back to the configured Aircall destination.

Quick Troubleshooting

IssueFix
Registration failsVerify Termination URI and SIP credentials. Check firewall rules for UDP/TCP 32681 or TLS 32682.
Calls not reaching SynthflowConfirm Smartflow is published and the SIP Forward step is active. Check Aircall SIP destination is enabled.
One-way or no audioEnsure RTP media ports are open bidirectionally. Confirm G.711 (PCMU/PCMA) codec on both sides.
Transfer failsAircall does not support SIP REFER. Synthflow initiates a new outbound SIP INVITE to the agent queue. Contact Synthflow support for configuration.
Caller ID missingSet the From header to the caller’s E.164 number. Verify Aircall passes ANI through the SIP INVITE.

Reference

  • Aircall SIP Integration: support.aircall.io
  • Aircall Smartflow Documentation: support.aircall.io/hc/en-us (search “Smartflows”)
  • Synthflow SIP Overview: docs.synthflow.ai/about-sip
  • Synthflow X-EI SIP Header: docs.synthflow.ai/x-ei-sip-header