Telephony

SIP Integration with Five9

SIP trunking is only available on the Synthflow Enterprise Plan.

This guide covers connecting a Five9 Virtual Contact Center (VCC) to Synthflow via SIP trunk for both inbound and outbound calling. Five9 can route calls to a Synthflow AI agent, and Synthflow can initiate outbound calls or transfer back to live Five9 agents.

Prerequisites

RequirementDetail
Five9Active VCC account with Admin access and SIP Trunking enabled
SynthflowEnterprise Plan with a configured AI assistant (v2.0+)
Phone NumberAt least one DID provisioned in Five9 (E.164 format)
NetworkFirewall allows SIP (UDP/TCP 32681 or TLS 32682) and RTP between Five9 and Synthflow

Five9 Configuration

All configuration is done in the Five9 VCC Administrator console.

Create the SIP Trunk

  1. Go to SIP Trunks → Add SIP Trunk.
  2. Select Generic in the Country dropdown, then choose Generic VoIP Provider.
  3. Enter the SIP trunk main number in E.164 format (e.g. +1XXXXXXXXXX). Click OK.
  4. Configure the trunk:
FieldValue
NameSynthflow AI
Registrar / Server / Gateway Hostname or IPsip.synthflow.ai
Number of Simultaneous CallsPer your license / expected concurrent volume
Type of AuthenticationIP-based (whitelist Synthflow IPs) or Digest
SIP Authentication IDFrom Synthflow — only required if using Digest auth
PasswordFrom Synthflow — only required if using Digest auth
  1. Click OK to save.

IP-based authentication is the simpler option — contact Synthflow support for the current signaling IP list and add those IPs to your Five9 allowlist. No credentials are needed. Digest (credential-based) auth is also supported if your Five9 configuration requires it.

Configure Inbound Calling (Caller → Five9 → Synthflow)

Route incoming calls from your Five9 IVR or skill group to the Synthflow AI agent.

  1. In your Five9 IVR script (or campaign skill), add a Transfer module.
  2. Set the transfer destination to the DID associated with your Synthflow agent.
  3. Select the Synthflow AI SIP trunk as the transfer path.
  4. Alternatively, configure a skill-based routing rule that sends specific call types or overflow calls to the Synthflow trunk.

For after-hours routing, use Five9 schedule-based IVR branching to route to the Synthflow trunk when the contact center is closed.

Configure Outbound Calling (Five9 → Synthflow / Synthflow → PSTN)

Outbound rules control how Five9 routes calls through SIP trunks.

  1. Go to Outbound Rules → Add.
  2. Name the rule (e.g. Route to Synthflow AI).
  3. Set matching criteria (dialed number prefix, skill group, user group, etc.).
  4. Select the Synthflow AI trunk as the destination. Save.

For Synthflow-initiated outbound calls (AI agent calling a customer), Synthflow sends a SIP INVITE through the trunk. Five9 routes the call to the PSTN via the outbound rule.

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 configured in Five9).
  4. Fill in the SIP connection details:
FieldValue
Termination URIYour Five9 SBC gateway IP or hostname (provided during trunk setup)
SIP UsernameYour SIP authentication username
SIP PasswordYour SIP authentication password
Outbound Proxy (if required)Five9 SBC IP:port (only if Five9 requires a specific proxy)

The Termination URI is the Five9 gateway address — this is where Synthflow sends outbound calls. It is not sip.synthflow.ai.

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 → Five9 VCC → IVR/Skill → SIP Trunk → Synthflow AI Agent
AI transfers to live agentSynthflow AI → SIP transfer → Five9 VCC → Agent Queue → Live Agent
Outbound from AISynthflow AI → SIP INVITE → Five9 SIP Trunk → PSTN → Callee
Outbound from Five9 via AIFive9 campaign/IVR → SIP Trunk → Synthflow AI → handles call

Verification

TestWhat to check
Inbound callCall the Five9 DID from an external phone. Verify it routes through the IVR to the Synthflow AI agent. Check two-way audio.
Transfer to live agentTrigger a transfer during a Synthflow call. Confirm the call lands in the correct Five9 queue with caller ID intact.
Outbound from Five9Route a Five9 IVR/campaign call to the Synthflow trunk. Verify the AI agent answers.

Quick Troubleshooting

IssueFix
Registration failsCheck Termination URI, credentials, and firewall (UDP/TCP 32681 or TLS 32682).
Calls not reaching SynthflowVerify Five9 outbound rule or IVR transfer points to the correct trunk. Confirm the trunk is enabled.
One-way or no audioEnsure RTP ports are open bidirectionally. Confirm G.711 codec on both sides.
Caller ID missingSet From header in E.164 format. Enable CNAM in Five9 VCC if needed.
302 errorsFive9 does not support 302 redirects. Use REFER or re-INVITE for transfers.
Call drops at 12 hoursFive9 enforces a 12-hour call limit on SIP trunks. This is by design.

Reference

  • Five9 SIP Trunking Self-Service: documentation.five9.com/bundle/sip-trunking-self-service/
  • Five9 Technical Requirements (IPs, ports): documentation.five9.com/bundle/technical-reqs/
  • Synthflow SIP Overview: docs.synthflow.ai/about-sip
  • Synthflow X-EI SIP Header: docs.synthflow.ai/x-ei-sip-header