Call transfer failures

Support Help
1

We’re having a call transfer failures (with 603 SIP error) on a consistent basis with multiple outbound agents. The agent recognizes the call transfer function and executes it correctly. After a few attempts it says the transfer was unsuccessful. The phone number was purchased from within Retell. Call transfer is setup to use User phone number as caller ID and its set to Cold Transfer. I’ve verified that the destination number is correctly formatted as E164 and we’re able to call and connect with the destination number manually. See few examples below:

Agent ID: agent_afdfda7ad854c6cf6cf3757a3c
Call ID: call_59a817af19def3acb9a5f64ea90
Detail Logs error message:
2026-01-24 17:21:50.901 call_59a817af19def3acb9a5f64ea90 error: Error transferring call via SIP REFER, SIP status code: 603 SIP error category: user_declined Error: twirp error unknown: call transfer failed: sip status: 603: Decline

Agent ID: agent_afdfda7ad854c6cf6cf3757a3c
Call ID: call_511591d83e9aa244d38817f63f9
Detail Logs error message:
2026-01-07 15:15:57.814 call_511591d83e9aa244d38817f63f9 error: Error transferring call via SIP REFER, SIP status code: undefined SIP error category: user_no_answer Error: twirp error unknown: request timed out

Agent ID: agent_68543a143daa1b4330fb3134df
Call ID: call_d84ceeb939920c5abf663403b83
Detail Logs: 2026-01-19 23:29:36.247 call_d84ceeb939920c5abf663403b83 error: Error transferring call, SIP status code: 603 SIP error category: user_declined Error: twirp error unknown: unexpected status from INVITE response: sip status: 603: Decline

Also, do I set the SIP transfer method as SIP REFER or SIP INVITE?

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2

The 603 SIP error (“Decline”) you’re seeing indicates the transfer destination is actively rejecting the call.(1)(2) This is different from a busy signal or timeout.

Caller ID Configuration Issue

Your setup uses “User’s Number” as the caller ID for cold transfers.(1)(2) For cold transfers specifically, Retell uses SIP REFER, and the caller ID override support depends on your telephony provider.(1)(2)

For Retell-purchased numbers (which use Twilio), cold transfers with caller ID override have known limitations. Retell Twilio numbers support showing the user’s number on both warm and cold transfer, while Retell Telnyx numbers only support this when using SIP REFER via cold transfer.(1)(2)

However, if caller ID override is not supported by the provider, the transfer will fail.(1)(2)

Recommended Solutions

Option 1: Switch to Warm Transfer

Warm transfers use SIP DIAL instead of SIP REFER and have better support for caller ID override.(1)(2) For warm transfers, Retell sets the from and P-Asserted-Identity headers to the user’s number.(1)(2)

To configure warm transfer:(1)(2)

  • Change transfer type to “Warm Transfer”
  • Configure on-hold music, human detection, and other warm transfer settings
  • Keep “User’s Number” as the caller ID setting

Option 2: Use Agent’s Number as Caller ID

Switch the caller ID setting to “Retell Agent’s number” instead of “User’s Number”.(1)(2) This eliminates the caller ID override requirement and should resolve the 603 errors.

Option 3: Add Custom SIP Headers

You can add custom SIP headers for outbound calls, which are forwarded to your SIP provider on the SIP INVITE.(1)(2) All header names must start with X-.(1)(2)

SIP Transfer Method

For cold transfers, Retell uses SIP REFER.(1)(2) For warm transfers, Retell uses SIP DIAL.(1)(2) You don’t manually set this - it’s determined by your transfer type selection.

The error messages confirm your cold transfers are using SIP REFER: “Error transferring call via SIP REFER”.(1)(2)

Given your consistent 603 errors with cold transfer + user’s number as caller ID, I recommend switching to either warm transfer or using the agent’s number as caller ID.

3

Both warm transfer and showing the agent number on caller ID won’t work for our use case. Do you think switching to a Retell (Telnyx) number could help resolve this since you mentioned Retell (Twilio) numbers have known limitations with cold transfers and caller ID override?

4

Unfortunately, switching to a Retell Telnyx number may not resolve your issue. According to the documentation, Retell Telnyx numbers only support showing the user’s number when using SIP REFER via cold transfer.(1)(2)

However, the documentation also states that Retell Twilio numbers support showing user’s number on both warm and cold transfer.(1)(2) This suggests your current Retell Twilio number should theoretically support your use case.

The Real Problem

The 603 “Decline” errors indicate the transfer destination is actively rejecting the incoming call.(1)(2) This could be happening because:

  1. The destination may be blocking calls from the user’s caller ID
  2. The destination may have spam filtering that’s rejecting the forwarded caller ID
  3. The caller ID override may not be working correctly with your telephony provider

The documentation warns: “If caller id override is not supported, the transfer would fail.”(1)(2)

What You Should Try

Before switching providers, I recommend:

  1. Test with Agent’s Number temporarily to confirm the destination accepts calls from your Retell number (this isolates whether it’s a caller ID issue)

  2. Contact Retell Support with your call IDs - since Retell Twilio numbers should support cold transfer with user’s caller ID, there may be a configuration issue

  3. Verify with the transfer destination that they’re not blocking or filtering the caller IDs you’re trying to present

Switching to Telnyx won’t provide additional cold transfer + caller ID override capabilities beyond what Twilio already offers through Retell.(1)(2)

**

5

in my call_transfer function settings I don’t have the “Displayed Caller ID” option to chose from

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6

Based on your screenshot showing the Transfer Call Tool configuration, you’re using the function calling approach (Single/Multi-Prompt with Transfer Call Tool), not the Conversation Flow approach.(1)

The “Displayed Caller ID” option you’re looking for is called “Configure Caller ID” in the Transfer Call Tool settings.(1)

Where to Find the Caller ID Setting

In your Transfer Call Tool configuration, you should see a section labeled “Configure Caller ID (Optional)”.(1) This setting allows you to choose between:

  1. Retell Agent’s number - The transfer destination will see the Retell agent’s number(1)
  2. User’s Number - The transfer destination will see the number of the user(1)

If you’re not seeing this option in your current interface, scroll down in the Transfer Call Tool settings panel - it appears after the “Configure Transfer Type” section.(1)

Important Note About Caller ID Override

When using “User’s Number” as the caller ID, the telephony provider must support caller ID override for this feature to work:(1)

  • For warm transfer, it uses SIP DIAL and sets from and P-Asserted-Identity headers to the user’s number(1)
  • For cold transfer, it uses SIP REFER, and it’s up to the telephony provider to support caller ID override for SIP REFER(1)
  • Retell Twilio numbers support showing user’s number on both warm and cold transfer(1)
  • Retell Telnyx numbers only support this when using SIP REFER via cold transfer(1)
  • If caller id override is not supported, the transfer would fail(1)

Given your 603 errors with cold transfers, this strongly suggests the caller ID override isn’t working properly with your Retell-purchased Twilio number. Try switching the “Configure Caller ID” setting to “Retell Agent’s number” to see if that resolves your transfer failures.