In certain situations you may need to resort to port forwarding on your firewall/router to get your phone working. If you are going to use port forwarding it is better to have a static IP address, if your not sure if you have a static IP address, ask your ISP.
For help on forwarding specific firewall/routers visit http://www.portforward.com/routers.htm, this page will also display your public IP.
Ports to forward
You may or may not have to forward all ports involved.
If your having problems with incoming calls going to voicemail you will need to forward to your local SIP port.
If your receiving calls but have one-way/no audio you will need to forward your local RTP ports.
The default port for SIP is 5060 but it may be different so check your phones configuration, it's usualy stored as local SIP port or Listen SIP port. If your phone is supporting multiple lines you may need to forward additional ports.
You will need to forward at least two ports, one for your media stream (voice) known as RTP and one for the media stream control, RTCP. There are no default ports for RTP they are normally in the range 1024-65535 so you will need to check you phones configuration, typicaly this port is called local RTP port or Listen RTP port in the phones configuration. The RTCP port is always the RTP + 1. For example if your local RTP port is 8000, then you need to forward port 8000 and 8001. Some phones use a range of RTP ports, in this case forward them all as random ports may be used.
If you are using a static IP address you can set your public
IP (also known as external IP) in your phones configuration (Do NOT do
this if you are using dynamic IP). This may help in certain situations. Your phones configuration page will
have an entry something like this Use NAT IP or NAT firewall IP (varies from manufacturer to manufacturer - refer to owners manual), this is where you configure your public IP.