Re: Roaming is completely knackered.
Not sure about calls when you're roaming. Is there some sort of contact every time - eg to see that you have credit or to bill you?
That depends if you're pre-pay (PAYG) or post-pay (normal contract).
Traditionally it was all post-pay so when you connect to a network, it checks with your home network if you're roaming enabled. If the SIM is roaming enabled, it will be allowed to make/receive calls/SMS/etc. The operator then sends CDRs in TAP files back home for billing (often few days behind).
That wouldn't really work for PAYG, so a more complex system (CAMEL) allows for real-time CDR exchange in which case, yes, there would be contact before each billable event (call,SMS,MMS, etc).