You’re right - it is a really unhelpful code! 
For a dial-up bearer, there’s a whole range of codes indicating specific causes like no answer, number busy, etc - but, for GPRS, there’s just this one amorphous code that covers absolutely anything and everything that could possibly go wrong prior to connecting to the APN.
I have just been complaining to Wavecom about this very thing!
As I understand it, the redult could be caused by any of the following:
Which, I think, boils down to:
- The SIM is not provisioned for GPRS;
- The network is GSM-only; it has no GPRS capability (do any of them still exist?) or, perhaps, the GPRS is out-of-order;
- The APN name does not exist;
- The APN name exists, but refuses the connection.
You should also check, of course, that you are GSM-registered before attempting to start GPRS…