Preserving Caller ID During SIP Transfers in LiveKit Agents

Overview

When performing a SIP transfer in LiveKit using TransferSIPParticipantRequest or CreateSIPParticipantRequest, the caller ID (from number) may default to the previous agent’s number instead of the original caller’s number.

This can be problematic in cold transfers and warm transfers, where the receiving agent needs to see the actual caller’s phone number.

Fortunately, you can preserve the original caller ID by passing it explicitly to the SIP request.

Solution

To preserve and pass the original caller’s phone number, set the sip_number field to the original from_number when making the CreateSIPParticipantRequest.

Example (Python Agent)

create_request = api.CreateSIPParticipantRequest(
    sip_trunk_id=sip_trunk_id,
    sip_call_to=target_phone_number,
    sip_number=original_from_number,   # <- Pass the original caller ID here
    room_name=room_name,
    participant_identity=supervisor_identity,
    play_dialtone=True,
    hide_phone_number=False,
    krisp_enabled=True,
    wait_until_answered=True,
)

Parameters Explained

• sip_number: This field determines what caller ID will be presented to the receiving party. By default, it may show the transferring agent’s number. Overriding it with the original caller’s number ensures the correct caller ID is displayed.

• sip_call_to: The destination phone number for the transfer.

• sip_trunk_id: The trunk to use for the outbound call.

• room_name: The LiveKit room associated with the session.

• participant_identity: Identity for the new SIP participant.

• play_dialtone, hide_phone_number, krisp_enabled, wait_until_answered: Optional flags for call behavior.

Notes & Considerations

• Ensure that your SIP trunk provider allows arbitrary caller ID (ANI) passthrough. Some providers may restrict the caller ID to verified numbers.

• If the SIP provider blocks unverified caller IDs, the call may fail or still show the trunk’s default caller ID.

• For warm transfers, the same approach applies — pass the original_from_number into the sip_number field when initiating the new participant.

Summary

To preserve the original caller ID during SIP transfers:

• Always set sip_number=original_from_number in CreateSIPParticipantRequest.

• Verify that your SIP trunk provider supports caller ID passthrough.

This ensures that the receiving agent sees the true caller’s phone number instead of the transferring agent’s number.