Another wild idea:

Instead of trying to determine where each phone is, we may just track wheter it has entered/exited the protected area.

Say you place a device near the entrance to the hall, like the ones there are in shopping malls to prevent shoplifting. This device picks the periodic signal from your phone, extracts the IMEI or whatever information is necessary and sends it to the provider. From this moment until you leave, your calls may be dealt with at the provider network -- special charges, operator voice warnings, call blocking, whatever.
The same device would 'log you off' as you walk out.

The device itself would be relatively simple, wouldn't it?

Of course this has a lot of loopholes :) But at least it should work with existing phones and does not need extra base stations.