You may find this article useful, if you are having issue when using multiple VoIP phones on the same network that’s connecting to an external PBX or VoIP server.
Most IP Phones have the local SIP port set at 5060 which is fine and will work perfectly fine most of the times, but you may run into an issuewith some routers which cannot properly route the traffic correctly when using several phones at the same time. The solution is to change the local SIP port, so that each phone uses a unique port number.
Using non-default local ports in your phone settings can avoid many of the most commonly encountered router and firewall related problems.
For two or more SIP terminals (VoIP phones) to function reliably behind the same router, each SIP Terminal should use different local UDP ports.
So above in normally the default port configuration for most VoIP phones and i highly recommend that you change that to something else. Below are some examples of some other ports that you can use and you can just continue following that pattern, for all the other phones that you have.
If your SIP device requires Start/Min or End/Max port values for RTP traffic, then you can just put a range similar to the pattern below.
The Local SIP Port is sometimes called the ‘SIP Signalling Port‘ or ‘Listen SIP Port‘.
The Local RTP Ports may be called the ‘Audio‘ or ‘Media‘ ports.
By changing the local SIP ports, this should resolve the issues you are probably facing with constant deregistration or unable to register in the first place.