In a Multiway call escalation, which SIP message is sent from the Multiway initiator to the participants to provide the bridge URI?

TRANSFER is not a defined SIP method; call transfer and escalation in SIP are accomplished using the REFER method, not a message called TRANSFER.

SIP REFER (RFC 3515) is the standard method an endpoint uses to instruct another SIP user agent to initiate a new transaction toward a provided URI. In Multiway escalation, the initiator sends REFER messages containing the conference bridge URI to each participant, triggering them to dial into the bridge and convert the point-to-point call into a multipoint conference. This is the defined signaling mechanism in Cisco's Multiway implementation on VCS.

REDIRECT (3xx) is a SIP response class issued by a server to redirect a request, not a participant-directed message that an endpoint sends to supply a bridge URI.

200 OK is a SIP success response to a previous request, not a request method capable of carrying a bridge URI to participants.