A detailed IMS subscriber to IMS subscriber is presented here. The preconditions for this call flow are:
- The calling IMS subscriber is currently roaming outside the home network.
- The called user is registered in the home network.
- The caller and the called user require resource reservation before the call can be setup.
The major steps in the call flow are:
- IMS routing of the initial SIP INVITE.
- IMS routing of the first response to the INVITE.
- Resource allocation via PDP context activation.