Reason 1: Network connection cut
The network connection may be cut or the Ethernet cable of your phone loosed. Your RJ45 phone cannot detect the disconnection immediately. Check your network environment and whether the Ethernet cable of your phone loosed. To solve, connect the Ethernet cable tidily and restart your phone or wait for approximate 5 mins after reconnect the Ethernet cable.
Reason 2: Firewall may exist in your network or you use network devices or software with
firewall functions. The firewall chops the communication.
Please first inquire your agent to get server IP, then contact your ISP or network administrator to cancel the following restrictions: the restriction that the connection from the phone IP to the TCP1730 and 1721 ports of server IP; the restriction that the connection from server IP to the TCP 1730 and 1721 ports of your phone IP.