By nature (for security reasons), firewalls block all but essential "ports" that your PC requires to browse the Internet. So, when you start installing VoIP softphones and VoIP hardware adapters (ATA), they often don't work because your firewall is blocking the "special" dedicated communication ports that the VoIP services need to communicate through.
Most people now days have NAT Routers and these routers also have built-in Firewalls.
So now, if your PC is behind a NAT Router, you may have two firewalls to traverse, if you are using a SoftPhone. First, your VoIP packets have to traverse the hardware NAT Router, then they have to traverse the PC software based firewall, such as Windows Firewall, or your pc security software like Norton, McAfee, etc.
Part 2 can be found here.