Excess charging by telecom company Prepaid for unused what to do

As I am a frequent traveler and travel for months I don’t used Airtel services regularly. The moment the account balance becomes zero they deactivate service. But when I come back & re-charge I see that money is deducted for the unused period. I requested for a solution for this & they said that I can activate the unused service period facility by paying a nominal fee of 30 Rs for a period of one month or 90 Rs for 3 months by which they don’t charge unused service period. But this can be done only once in 4 months. They don’t offer this service of temporary deactivation as on need basis and forcefully collect payment for months I don’t need service. How can I proceed legally? Hope they are earning lots of money for service not provided to many customers they way they are doing it with me. Please help correct this practice which may benefit many consumers across the country.