HT818 Not ringing calls through FXS ports


I am a service provider and we have deployed a HT818 ATA to provide multiple analog lines over a SIP connection. We have no issues getting the lines up, registered and calling in both directions. However, periodically, (up to several times a day) the PBX the FXS ports are connected to, stops getting dial tone and also receiving the ringing for incoming calls. A reboot temporarily fixes the issue but it repeats and the only correction is to reboot the Grandstream again.

We are running firmware


where is the ht located in relation to the pbx, local or remote?