Open Source Foip Solution

I love PuTTY !

The PuTTY executables and source code are distributed under the MIT licence, which is similar in effect to the BSD licence. (This licence is Open Source certified and complies with the Debian Free Software Guidelines.)

PFsense can do that. To be quite honest, any firewall should be able to pass SIP and RTP through untouched.

SIP is only the mechanism for initiating calls, so that's only part of the problem. RTP is the protocol for the voice traffic itself. Some VoIP phones can use UPnP to talk to the firewall, and dynamically configure port forwardings as they make the calls.

PFsense can also do QoS, but I'm not sure how well it implements it (I've been configuring PFsense at work lately, and haven't got to QoS yet!)

This might be an interesting read, NAT + VoIP (From VoIP-info.org). The thing you probably do want to do is to restrict the SIP connections inbound to only come from your provider's SIP gateway, otherwise nasty people tend to connect in, and make expensive phonecalls on your account. Make sure you also lock down other ports to the phones like HTTP(S) and Telnet. I found an interesting security hole in a company once by being able to telnet into a public SIP phone, then ssh out onto the network.

You should also consider having a separate VLAN for Voice traffic. This can help with QoS, and eliminating jitter.

These might be interesting to have a look at regarding multiple SIP registrations and PFSense