TLDR: People who travel to India couple of times a year keep one Indian SIM card with them. To keep the same number active, they top-up the number once a year for Rs. 200. Pretty easy to top-up online if you have an Indian bank card. Not easy if you don't. Big websites I've seen (Freecharge, PayTM, etc.) don't take international bank cards.
Which website to use to top-up an Indian mobile number with a non-Indian bank card?
I don't want to bother my folks in India to do it for me.