I have configured a GrandStream Handytone-488; POTS (FXO) works fine, and the FXS port on it can call other extensions. Problem is, if I dial on the outside line from the FXS, I get a busy tone returned. I tried using # to send and then taking it out; nothing. I know my routing is correct for PSTN because I can do it from any phone (iPhone, softphone) without any problem. Also, will the FXS port always ring no matter what?