IMO is best calling app.
It so simple and easy to use.
I’m using it from long time. It is one of the famous app. It is available for android, iOS and windows too. Its free of cost to download from app store.
I’m using to call my family and friends. Its size is not big too. It also give facility to send photo videos. The video call’s clarity is very impressive on it. It works very good on 2g network too. The design is very simple. If you want to search anybody on IMO then you can search him too its the best feature I think. Overall its best application for vidocall and voice call.