Cisco IP phones are the worst phones to be used with our system.
Perhaps they are ‘the best’ phones in your mind, but apart from having the ‘Cisco’ brand name and good hardware, the software is nothing but a problem.
We do have customers using this phone, but they control their network fully.
The only way this phone might work in your setup is to use SIP TCP, but this is not tested and you may end up having some issues. SIP TCP means that the phone will have the connection open with the server all the time, unlike UDP where there is no connection between the phone and the server.
There was a generation of Cisco phones that used to work well with SIP UDP and that is Cisco 79X0 phones – 7940, 7960. Those phones had a setting ‘nat_enable’, and did work for what it was supposed to.
The latest generation of phones do not work like that and they go to SIP TCP by default. We couldn’t support SIP TCP until the latest version of our software, v3.8, hence most of our customers still use UDP.
Try the the following for one phone and one phone only (SIP UDP):
– On your NAT box/firewall make sure that SIP UDP port 5060 points to your phone
– Try registering to our system with nat=no (This is important! Do not let the name confuse you, it has nothing to do with the fact you are behind NAT)
– That should work
SIP TCP requires some settings to be turned on and template to be customized to force the phone into TCP mode.