It's important to remember that voice quality depends on the connection strength on both sides of the conversation.
Sometimes your wireless reception could be low, and this is something we cannot control. Naturally this affects Viber's call quality and results in disruptions and broken sound.
We are continuously trying to improve our handling of broken/overloaded networks, so you can expect better performance as newer versions of Viber are released.
Please note that having full signal still doesn't mean your internet's bandwidth is at the maximum expected. When you see a "full reception bar", it merely means that physically, your phone is covered with the cellular signal.
When it comes to the bandwidth itself, your connection could still be slow due to various different reasons, such as being in an area with many other users from the same provider, thus congesting the network.