I have a problem for received calls form my Grandstream HT-503. I have a FXO connect to my PABX, and I can make a call from PABX to VOIP, but I didn’t received calls to my VOIP, to my PABX.

Any ideas of what might be happening? this is the type of device: Grandstream HT-503

Note: the HT503 is an analog telephone adapter featuring 1 analog telephone FXS port and 1 PSTN line FXO port. The integration of a FXO and FXS port enables remote call origination and termination to and from the PSTN line, known as “hop-on and hop-off” calling. The 1 FXS port allows for extension of a VoIP service to 1 analog phone. In addition to being SIP compliant, the product features support for up to 2 SIP account profiles, excellent voice quality and advanced telephony features