What would be useful is a daemon process that monitors an active GSM session, and depending on the dialed number invokes a rule to hang up / warn the user after a certain time.

A specific case here could be VFUK's "Stop the Clock" (see http://www.vodafone.co.uk/vodafone-u...lock/index.htm).

So, just to expand upon this, a Call Daemon that allows for plugins that could be used to work for network packages. One such plugin would be "Stop the Clock" with either the option to warn or indeed hangup after 57mins.