[UPDATE] Probable Solution found
My phone (iPhone 7) mic stopped working as you guys can figure out.
Coincidently I signed out of my Apple account and signed in again using another Apple ID. And as soon as I did that, the mic started working again!
Facetime works, Siri works, Video calls on WhatsApp works, Voice memos work and most importantly, I can make phone calls again.
I don't know how it fixed the problem but it might have something to do with the associated Apple ID or whatever.
Earlier I had reset the phone in every possible way but that didn't solve the problem as well, but now, it has.
If that can fix it, it's worth a try. So, give it a try before buying a new phone.
-Go to settings and sign out.
-Use another Apple ID to sign-in
-Check if its working (I spent at least 30 mins using the new Apple ID before noticing the problem was solved)
-If it works, you can continue using this new ID or sign-out from this and sign-in using your older ID
This could be a coincidence as well. And who knows the problem might start again.
But as of now, it's working fine. I will update if the situation changes.