Cellular connections can be terrible in remote areas. I'm sure there are times when you notice that it's still goes crazy midday right in the CBD or at home and you have no idea why.
This is because cell signals can easily be disrupted and it gets worse when out in the wild.
For the most part, its possible that your Viber is registering his as ringing but the whole time it's not even showing on his mobile. Or he could already answered your call (but yours is still showing Ringing) and since it wasn't actually going through, he cut the line but the network hung his app connection. This may show him being online even when he his phone is far away.
Also, I've found that sometimes, I receive messages but when I reply them, they don't send for up to hours and by some stroke of bad luck, it will be happening to just one contact if not more.
Anyway, I hope this answer puts you at ease somewhat and all goes well.
PS: You could just try an actual call, if you can.
All the best.