Airtel prepaid travel ripoff or settings issue (Bangalore)

Hi Internationals!

Wondering if there is local area knowledge on this...

Some of our mobiles are on Airtel. Whenever we travel out of the local area, by car or plane, with phone left on or switched off then on at destination, Airtel drains the prepaid balance within minutes, requiring a recharge.

If anyone knows what causes this and how to keep balance when travelling, please either reply directly outside this group or if ok, within the group.

It seems like an Airtel ripoff but maybe there are settings or techniques we are supposed to know about when using the Indian prepaid mobile system...

Many thanks, David