This is by design. Think of it as a kind of Fair Usage Policy. Even Uninor has a similar 60-minute 'Fair Usage Policy' in place.
I think this is not a fair use policy but required for billing purposes (and a few other reasons).
let me try to explain with an example. You have 200 minutes free in a billing cycle and your billing cycle starts on begin of 15th of the month.
You initiate a call at say 2040hrs of 14th of the month
now imagine you talked for 400 minutes
and disconnected.
Technically you used 200 minutes (2040hrs to 2400hrs) of the billing cycle
and 200 minutes of the new cycle.
however how to the billing happen? how do the settlement happen?
this is just you, imagine if the call was to another operater. then the whole cycle of inter operator billng etc also need to happen. (classic problem that many people face on BSNL and AIRTEL fair use policy internet plans)
Thanks to the batch processing and multiple layer of systems nothing happens on instant... a call that you made today using mobile might hit billing system in a minute or even a few days. this is the reason why billing in post paid does not happen instantly. Yeah now people will say what about the prepaid. in prepaid it is just the balance deduction that happens instantly all other things like inter operator billing, CDR log retention etc would take time.
lastly, from the customer perspective, imagine you forgot to lock the phone (or say both parties forgot to hang the phone), imagine if this feature is not there what would be the cost of that call.
Lastly there are many internal timers in the system, the GSM technology is OLD... check the processing power of year 1995-2000 when GSM came.. maybe the protocol is such that the timer beyond that figure is not possible and hence all the operators have some conservative cutoff (ever tried booting a system,
android phone, camera which have its primary storage full... no one can guarantee you a safe experience)
just my 2 cents, not sure what is the actual reason, and i too some time do not like it especially when you have to attend over 2 hours of conference call from your mobile.