Do airtel numbers which are deactivated due to non usage gets reactivated again in future or will be permanently disabled?

Google shows "Once the sim is deactivated, the same number is recycled and becomes available in the market for a new user" But a number (airtel) which was owned by my sister a long ago like 12+ years isn't still reactivated after it got deactivated in 2015-16 . I actually own an airtel prepaid number but it got deactivated due to non usage and i contacted local airtel head office and they said that the only way to get your number back is to convert it to postpaid , but my father said that will get me into unnecessary issues and bills like they charge me even if i don't use it as my dad said but i don't really know and also my primary number is jio and i only use airtel as it was linked to my whatsapp long ago and recently my whatsapp account got logged out itself, i still regret that i didn't change the number before. My question is, will my airtel prepaid number gets recycled/reactivated and can be used by new users or will it be disabled permanently? And is there any other way to login to my whatsapp?