SIP REFER
SIP REFER is a signaling method that asks a SIP endpoint to initiate a new request to a target identified in the Refer-To header. In voice systems, it is commonly used to request that an active call be transferred to another person, queue, or service.
How SIP REFER works
One participant sends a REFER request during a SIP dialog and names the transfer target. The receiving endpoint decides whether to accept the request and, if it does, attempts the new SIP request. Status notifications can report the progress and outcome of that attempt to the referring party.
Acceptance of the REFER request is not the same as a completed transfer. It means the recipient accepted responsibility for trying the referred action. The target may still be busy, unreachable, rejected by policy, or sent to voicemail. A reliable call flow must handle those outcomes instead of assuming that the original caller is safely connected.
REFER is often associated with a blind, or cold, transfer, where the original party sends the caller toward a destination without first speaking privately to the recipient. An attended transfer involves a separate consultation call before the parties are joined. SIP systems can implement transfer flows in different ways, so interoperability depends on the PBX, carrier, application, and supported SIP behavior at each boundary.
Why it matters for AI phone calls
Transfer is a critical transition between automation and human service. The voice agent needs to choose an allowed destination, preserve the caller's experience while signaling proceeds, and respond appropriately if the target does not answer. Transfer policy should define business hours, fallback destinations, voicemail handling, and what the agent says before and after an unsuccessful attempt.
Testing should cover successful transfers as well as rejection, timeout, busy, and voicemail paths. Teams should verify what caller ID the recipient sees, whether audio remains intact, when the original call leg ends, and which system records the final outcome.
In practice on ThunderPhone
ThunderPhone supports cold and attended warm transfer. In warm mode, the caller waits on hold while the agent privately briefs the recipient, then connects the call after acceptance. If the recipient declines, does not answer, or reaches voicemail, the agent returns to the caller with the outcome. Warm transfer requires an eligible production number that can place an outbound SIP leg; demo numbers and browser-based calls are limited to cold transfer.