Traffic-wise, that is.
Cumulated traffic, about 41 MB for 42 minutes. Let’s say 1 MB/minute. That’s 8 Mbit/60 s, or about 136 kbps.
If you’re having an “unlimited” data plan that caps the speed at 128 kbps or 64 kbps after a certain monthly traffic, you really need Wi-Fi for Viber to work.
Otherwise, even EDGE should be able to offer 236 kbps, you shouldn’t need 3G to have Viber working with voice.