Just to add to what has already been said, understand this is a networking issue, not a 3CX issue. When you plug in a new phone to your network, the phone makes a broadcast saying essentially "hey, I'm a phone. Is there a PBX out there listing?". However, most modern switches are configured NOT to propagate broadcasts to other switches (to avoid broadcast storms). So in truth, the visibility of this broadcast is actually not just limited by subnet (because you could have many switches on the same subnet), but is actually often times limited to just the switch. Therefore, if your phone and 3CX server are plugged into separate switches, that alone may be enough to cause the phones not to show up. I concur with Eddie. Set DHCP option 66 and then your phone knows where the PBX is, rather than having to rely on a broadcast propagating through your network to find the server.
