R1.   Registration: user not registered R2.   Re-registration R3.   UE subscription for registration state R4.   P-CSCF subscription for registration state R5.   Deregistration in S-CSCF R6.   Deregistration in HSS R7.   Deregistration upon UE roaming and registration to a new network R8.   Re-authentication S1.   Session-Initiation: mobile origination and termination roaming, with different network operators S2.   Session-Initiation: mobile origination and termination in home network S3.   Session-Initiation: origination in CS Network, and mobile termination in home network S4.   Session-Initiation: mobile origination in home network, termination in CS network S5.   Session-Initiation: mobile origination and termination in home network, but terminating UE roaming and busy