I believe that a PPP session only exists if you want it to exists: whem the mobile attaches to the GPRS network (through SGSN node) and activates the PDP context, it gets a mobile IP that id's the MT in the GPRS network... Then it can dial-up the remote network (there'll be a dial string and the subsequent CONNECT, PPP session, etc.). But before you can dial a remote network using the GPRS backbone you must activate a PDP (it's the routing mechanism that supports the very GPRS network model). Hear this: whwn you dial-up using GPRS, the mobile will AUTOMATTICALLY attach to the GPRS network and activate the PDP (if any), and only then dial the network...
So, resuming:
1. attach to the GPRS network
2. activate a PDP context (it routes the IP traffic to the APN that supports your SIM card)
3. dial and establishe a PPP tunnel