we have a case in which we have installed the 3CX server in a main office with 5 Grandstream GXP 2000 IP Phones , and we have 6 branches and we need to put one Grandstream GXP 2000 Ip phone at each branch to be connected and registered on the 3CX server at the main office which have a static IP . But the remote Hard phones does not work ?? what is the problem and what should we do with that ? and what about outbound proxy??