Let's say that Alice would like to make SIP call to Bob. Alice initiates the call using her mobile device over a cellular network. On the other hand, Bob sits in a local network of an organisation. The traffic should go from Alice, then to Bob's organisation SIP server/gateway, which in turns routes the call to Bob's end device. It's a requirement that the traffic on the leg between Alice's mobile device and Bob's organisation SIP server/gateway is encrypted with TLS/SRTP. Therefore the SIP server/gateway needs to decrypt any incoming SRTP traffic to RTP traffic, before routing it to Bob's end device. Obviously, it also needs to encrypt any outgoing RTP traffic to SRTP for the other direction.
What is the exact name or term of this SIP server/gateway?