I have a GrandStream HT-502 with the latest firmware.
Right now my cable modem is connected directly in the WAN of the GrandStream and I have configured the FXS PORT1 with the right infos I think or maybe not

The issue is simple. Randomly when we make a call we sometimes not hear any "ring tone" but the call is working when the other person take the call. I can hear and he can hear me well.
Any idea how to fix this random issue ?
Thanks !