first you need to install the SIP stack onto your phone, if it doesn't have one. then your SIS must include an installer thing, look at the pkg file from example (for SIP plug in 1.0/2.0). third, you need to have a public IP on your phone in order to connect back to your SIP server; otherwise the SIP server will remember a fake IP of your phone, and INVITE will not work
First, is your 6620 connecting to the network? If so, the little G under the signal strength indicator will have a square around it.
Second, where is your server? If you're using the server emulator, it's probably behind a firewall, which usually can't handle SIP messages.
Either way, the SIP stack on the 6620 is(probably) not going to recognize the registration. The SIP stack will reject the 200 OK message from the server if the address in the SIP message has been translated.