First, check if the problem occurs all the time or at certain hours.
Now proceed to the SET tab and check if there is call blocking or forwarding, in general:
Or at Out-of-Office hours:
In addition, check whether the application has a “White List” mode – a mode in which all incoming calls are blocked, except for calls from specific numbers. To cancel the setting, and switch to receiving calls from all numbers, switch to “Black list” mode.