sorry I was in hurry, as I provided too little info.
Before I used fix line and it was working perfectly.
But when I connect GSM wireless modem to A800, seems like A800 is unable to detect ring tone when I dial to the GSM modem. (the PSTN port suppose to be light on but in this case it doesn’t)
So I thought maybe the GSM modem doesn’t have enough voltage to perform the line reversal, and I check the line reversal voltage, it looks normal too.
so now I am thinking maybe I need to set ring tone to different frequency or something.
Does anyone know how to solve this problem ? thanks