I have a customer who needs to connect his two branches PABX through a VOIP such that a user can just dial an extension to reach the other side, his PABX does not support E&M signaling, and he need the call to be done from both sides. should i use FXS to FXS configuration, and connect both sides to CO lines at the PABX, or FXS to FXO is better as I know that FXO- FXO is not supported.