For SIP trunk connections to third-party devices, such as SIP-based PBXs or service-provider IP PSTN connections, Cisco recommends the use of Delayed Offer for outbound calls from Unified CM and either Delayed Offer or Early Offer for inbound calls to Unified CM.SIP trunks can use either TCP or UDP as a message transport protocol.Fax gateway can be registered as a SIP gateway and use the configuration of MTA.