If a customer cannot receive incoming calls after a certain time being idle, then it is most likely that the firewall is configured with a UDP port closing timer. 

Probably are incoming calls possible after making an outbound call. If this is the case, the firewall should be checked on the settings of the UDP port closing timer. 

We advise to configure the registration time in the PBX or device between 60 to 300 seconds. In the firewall the UDP port closing timer must be higher than the registration time of the device to keep the ports open.