Connect Retell to Bitcall (Outbound SIP Trunk)

Use Bitcall as the outbound SIP trunk for your Retell AI voice agent. Step-by-step elastic SIP trunking setup: termination URI, auth, caller ID, and a test outbound call.

June 4, 2026

4 min read

Connect Retell to Bitcall (Outbound SIP Trunk)

Before you begin

💡 Heads-up: Bitcall is outbound-only today — great for Retell agents that make calls. Retell uses two trunks (one inbound, one outbound); this guide sets up the outbound one. Inbound numbers (DIDs) are coming soon.

"Retell brings the brains. Bitcall brings the dial tone — outbound, worldwide." 🤖🌍

🧰 What You'll Need

A Retell account with dashboard access
A Bitcall account with a SIP account (username + password) — see Set Up Your First SIP Account
An outbound Caller ID (CLI) you're allowed to present
A little Bitcall balance (prepaid)

🔌 Step 1: Know the two sides

	Value
Your trunk (Bitcall) — termination/SIP server	`gateway.bitcall.io`
Port	`5060`
Auth	SIP username/password or Trusted IP
Codecs Retell supports	PCMU & PCMA (G.711), G.722
Encryption	SRTP — set transport to TLS to use it

Retell connects to your carrier with elastic SIP trunking (its recommended method). You'll give Retell your termination URI (your Bitcall gateway) so the agent can dial out through Bitcall.

➕ Step 2: Add Your Trunk in Retell (Outbound)

In Retell, open Phone Numbers → click "+" → choose Connect via SIP trunking (elastic SIP trunking).
Add a number for outbound calling and enter:
- Termination URI (your SIP server): gateway.bitcall.io (port 5060)
- Username / Password: your Bitcall SIP credentials
Save.

⚠️ Retell won't validate the setup until a call is actually placed — so a failed save isn't the test; the test call is.

🔐 Step 3: Choose How Bitcall Authenticates Retell

Pick one (see Trusted IP vs. SIP Credentials):

Option A — SIP Credentials (simplest): the username/password you entered above. Recommended for most setups.

Option B — Trusted IP: whitelist Retell's SBC IP blocks in Bitcall so traffic from Retell is accepted without a password:

18.98.16.120/30 (all regions)
143.223.88.0/21 (certain US traffic)
161.115.160.0/19 (certain US traffic)

🎭 Step 4: Set Your Caller ID (CLI)

The number you added in Retell is your outbound caller ID (not a Bitcall DID — Bitcall is outbound-only today). Caller ID behavior depends on the destination country and operator rules. Manage it here: Change or Manage Your Caller ID.

🚀 Step 5: Place a Test Outbound Call

Trigger an outbound call from your Retell agent (dashboard or API). 📞 The agent now dials out through Bitcall. 🎉

Transfers (optional): Retell supports SIP REFER. To cold-transfer to a PSTN number with the transferee's caller ID, enable SIP REFER / PSTN transfer on your trunk.

🛠️ Troubleshooting Table

Problem	Likely cause & fix
Outbound call not connecting	Check the termination URI is `gateway.bitcall.io` and your credentials are correct. Check your Bitcall balance and that the route is enabled.
`401` / `407` rejected	Wrong SIP username/password — or, if using Trusted IP, Retell's SBC IP blocks aren't whitelisted in Bitcall.
Connects but agent is silent / one-way audio	Media path issue. For SRTP, set transport to TLS. See AI voice calls dropping after 30 seconds for the RTP/firewall fixes.
Call drops after ~30–60 seconds	RTP inactivity during the agent's thinking gaps → enable RTP keepalives, raise the RTP timeout. Full guide: AI voice calls dropping after 30 seconds.
Caller ID wrong / missing	CLI rules vary by destination. See Change or Manage Your Caller ID.

🧠 TL;DR Recap

✅ Connect via elastic SIP trunking in Retell → Phone Numbers ✅ Termination URI = gateway.bitcall.io:5060, with SIP username/password ✅ Auth = SIP credentials or whitelist Retell's IP blocks (Trusted IP) ✅ Set your outbound Caller ID, top up, place a test call ☕

AI Voice Calls Dropping After 30 Seconds? The RTP Keepalive Fix

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