In either case, whether you are using Windows Firewall, or a third party software firewall, they will typically prompt you any time (or at least the 1st time) an application on your PC is trying to access the Internet. At which time, you will need to make a decision to "allow" or "disallow" the program to access the Internet. Once you tell Windows Firewall, or the third party software firewall that the application is "safe" to access the Internet, you will no longer be prompted when that application (i.e. your softphone, Skype, Google Talk, etc.) is accessing the Internet in the future.
However, if you use an external hardware firewall, which is typically built into your NAT Router, then applications or any other hardware on your LAN (Local Area Network) which requires special ports to be open will not function properly... unless you know how to open the specific "incoming" ports required on the NAT Router.
SIP based (Session Initiation Protocol) VoIP services typically use standard ports that must be allowed to traverse your firewalls. SIP is the standard protocol that sets up (initiates) your SIP VoIP calls over the Internet. SIP protocol typically use SIP ports in the range of 5060 - 5080. However, in 99% of all cases, you will only require that ports 5060-5061 be open. (Unless you are using a multi-line IP-Phone, which may require more ports for each additional line used.)
That said, we're not done yet with opening ports for VoIP. SIP based VoIP calls also require RTP ports (Real-time Transport Protocol) to be opened. RTP is the protocol that actually transports your voice packets. So, SIP data packets set up the call and control, but RTP data packets carry your actual voice information.
VoIP RTP ports typically fall in the range between 10,000 - 20,000. So, you will be required to open some if not all the ports in this range to support your SIP VoIP calls.
Cisco Linksys VoIP ATA phone adapters typically default to requiring RTP ports 16,384 - 16,482. So, this is typically the range that I personally set to open in my router firewall.
Typically, there are two ways of opening ports on your NAT Router:
- Port Triggering
- Port Forwarding
Part 1 can be found here.