A lot of people who work in those call centres are uneducated and don't know shit about tech, they're only parroting off a script. In those call centres they use windows 7 still and don't even know about the existence of linux. Also piracy is big in India so they don't care if linux is free. And most of all their victims/"customers"(old people) don't use linux so they have no reason to either. It's just the fellow Indian nerds using Linux.