Introduction
If the User can't receive calls but can call the others, then you have a very specific Incident to deal with. This kind of Incident is very often a configuration mismatch from client to server about, for example, the PrivateGSM is configured as a Professional version, instead it is an Enterprise. Other bogus configuration has to be checked as well. As the PrivateGSM can place the outgoing calls, it's unlike that the cause of the Incident is a network issue. However we better verify any possibility before jumping to the conclusions.
User can't receive calls
First Level
In order to test investigate this kind of issues, the user has to call the customer service with a device different from that on PrivateGSM is installed.
...
If the call goes fine, let's try to call the user to determine if he (she) is actually experiencing problems to receive calls. If the call fails, first check that the PrivateGSM configurations are correct and if not, fix them and close the Incident. If they are fine, let's focus on the other party configuration, in most cases the caller's device is registered on another server then we can close the incident as "Not an issue", if the server is the same for both of devices, we need to escalate to the second level.
Second Level
...
If all of the above are fine, the final possibility of a bug in the client become feasible, thus we check the client logs and then we escalate to the third level.