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.