This is normally caused because of not-sufficient internet speed.
How many phonesets/lines are you using at the same time, on the same internet connection?
For normal internet, run some speed tests (google speed test, bing speed test, etc) and confirm upload AND download speed - usually upload speeds are too low.
For cell-mobile, because of movement it is very hard to ensure internet speeds are consistently fast enough. For our mobile users, we always setup and use the call-out feature, which uses the RC app to call out so that the recipient sees our number, but the actual call once connected is through the actual cell phone, not using any internet at all. You can also do this from a computer using the windows/mac apps.