You don't need a multi-platform solution. You need a SIP client for whatever platform YOU use. Let everybody else worry about their own platforms. There are TONS of SIP clients. Ekiga is one (which, yes, supports VIDEO)... and yes, ekiga DOES offer free accounts... and IS multi-platform.
NO, SIP does NOT require you to even HAVE an account with any retarded provider. It happens to also be perfectly happy running PEER-TO-PEER, so you know the IP address of whoever you're calling? Or their DDNS address? Then dial their IP.
And no, you don't need to know anything about NAT. At least not when you're using a registrar OR a router with phone ports (which obviously understands SIP and deals with your routing).
Edit: Free SIP account:
https://www.ekiga.net/?page=register