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.