Imo app is best all over app available because its dealing with less data consumption and its also used this 2G network.iwho its a good quality ever because other app coud not connect with 2 G network.so its good for using 2G network while with low quality video imaging.
But I using 3G with network so I want a good quality video calling compare to 2G network.
Its have also supported audio calling with out interrupt voice over 2G network.Not noise nor break sound.so its for audio calling app is super other than.
in whats app Voice calling also breaking with 2G network but in IMO not breaking .voice.