After the registration, the user is able to access the services and starts session establishment process. This process is briefly explained as follows and depicted in figure

• The Caller’s UE (UE-A) constructs an INVITE request that includes registered public user identity of called user (UE-B).
• All SIP messages must traverse the Proxy Call Session Control Function (PCSCFs and the Serving-CSCF (S-CSCF) of both users.
• All SIP messages are sent via the established IP Security (IPsec) security associations (ASs) between the UE and their P-CSCFs.
• All SIP messages are sent compressed between the UE and their P-CSCFs
• The UE-A and UE-B agree on the media streams that they will exchange.
• The UE-A and UE-B agree on a single codec for every media stream that they will exchange.
• The networks will authorize the media for the session, so that the users can reserve the related resources.
• The UE-A and UE-B perform resource reservation i.e. they set up the necessary media PDP contexts over which the media streams to and from the network will be transported.
• The network elements will exchange charging information.
• The S-CSCFs may initiate advanced services for their served users.
• Finally UE-B starts ringing and the called user accepts the session. This completes the session establishment phase