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Does this problem occur on both 3G and WiFi? or only on 3G (the cellular network) ?
We're asking this because some mobile operators across the world block Viber on their 3G network. Usually this happens when users have a "simple" data plan (or a prepaid contract), meaning they didn't pay extra for using VoIP on their 3G network.
In order to verify this, we recommend to:
1. Test Viber on a few different WiFi networks, and see that it works fine.
2. Contact your cell company and confirm this with them.
Have you got this question resolved ever since? I've got the same problem with my iphone 5S. I've bought it a month ago as part of a 2 year contract with Orange France. My plan has 2Gb data on 4G. But only when I'm on wifi does any of those work (viber, facetime, skype). Once on data plan, no matter where I am in Paris, I can't hear people answering my facetime/skype/viber calls and they can't hear me either. Even though the status shows 4G (or sometimes 3G). I've tried disabling 4G and forcing 3G, the same. I've got the iphone replaced, then the SIM card replaced, the problem stays. I still have more than 1Gb left from my data so that's certainly not the problem. All softwares are updated (certainly downloaded, re-installed everything several times, phone factory reset, etc). I pay like 45 euros a month so I can't imagine this would be a 'simple plan' or maybe my VoIP got somehow disabled by mistake? I've been to 5 Orange stores and they sent me away that it must be application problem they don't know what's wrong. As I said, Apple got my iphone even replaced but didn't have more suggestions when it still didn't work. I'm kinda hopeless, doomed to go for 2 years with an iPhone 5S that can't use any audio communication applications from its data plan. Which is ridiculous cause that's one of the main reason I've bought a 5S, I was told this one can do Facetime on data plan too, not just on wifi.