Problem: The 3CXphone software will not connect with the 3CX server in CTI mode. Configuration: Setup is vanilla v12.5 with license. System working fine with mixture of Yealink and Cisco handsets using SIP trunks. We have recently moved IP address range from 192.x.x.x to 10.x.x.x Server/Phones/Client computers all on same 10.x.x.x subnet Clients can ping and access shared files on 3CX machine using standard file sharing. Firewall is OFF on both server and clients (for testing). 3CXPhone is 12.5.42341.0, Server is 12.5 with service pack 1. Client software has been configured using the Welcome Email file. Firewall checker in 3CX passes. In Network the internal IP is correctly set to 10.0.0.21, external IP is set to a FQDN that resolves to the correct external IP of our broadband feed. The client is trying to connect, if I deliberately make the passwork in 3cxphone wrong, then it errors with "Invalid Password" so must be able to talk to the server. In Custom Parameters I note that MYPHONEVERSION is 12.5.41543.0, this is different to the client version. I have tried changing this but as yet have been unable to restart the server services to see if it makes a difference. I note in 3CXPhone Clients in the web gui that my desktop phone extension tries to register multiple times. It displays the IP of the client software as 10.0.0.50 sometimes and others uses the External IP of our connection 82.x.y.z, same as configured in Settings>Network for the external public FQDN when resolved. After a while, the app throws the "3CX Phone System is unreachable. 3CXPhone will try to reconnect automatically. Please wait." error. I've noticed that two users with the older version of the software 12.5.415 *ARE* successfully using the software. I'd welcome any suggestions, I am unable to find the log in the client software, can anyone direct me?