I don't have a 6620 but I'm buried deep in Voip-city muck with my 5530 right now so I hope I can help. Actually, you already helped me by inspiring me to open up this broken feature on my phone and check it. While looking around I made an accidental call, and low and behold, it was successful! I couldn't believe it. All the trouble I went through wasn't the phone's fault. (Stupid Google Voice.) I am soooooooo happy! Thank you.
Now to pay you back, hopefully. Here's hoping your sip settings are similar to mine...
If you get the native Sip settings working in your phone it actually turns your contacts folder into a better Voip software client than most I've seen. Much better than Fring! The main reason is that there is no address book syncing needed. You just use your real one, with call history and all.
After you've set it up (in Connectivity settings) just go into your contacts and you will see a new tab at the top, with a little blue and green globe icon. Open it. If it says Login then you need to open it and follow the prompts, if not then you're up and running already.
Go back to the main address book tab. Now when you open a contact you'll see a new "Internet Call" option listed after "Voice Call." (Don't press the green call button. Just open the contact.) You don't need to copy the contacts into the Sip tab as far as I can tell. My phone calls them just fine this way. There may be other features I haven't found yet when you put them over there, who knows? I can see that it does video calls for those with front cameras. If I find anymore goodies I'll share them.
Tip: If you remove your SIM card the Sip service will now be your default phone -- from green call button. (You need wifi of course.) Your phone will squawk about this but it should still work fine. Just edit the Offline profile so your ringer is not silent. (Great use for a second phone.)
Tip2: The sip client will always be trying to connect to the Internet. It doesn't hog resources by itself, but unless you turn your wifi auto-scanning off it will trigger it to keep trying to find a network away from home, 24 hours a day. This tends to get the battery pretty upset so he shuts down on you, just when you need him the most. Instead, I login to the Sip manually when I am plugged in at home. I think there are settings to tweak this too if you look around.
Tip3: I have a Google Voice app on my desktop called GVnotifier that alerts me about new calls even if my phone is not connected to the wifi. It also sends outgoing Google Voice calls to my cell phone's SIP without an Internet browser. Free calls through Sipgate.com or Callcentric.com: check it out.
If you need SIP configuration help there are some nice guides for Nokia in the FAQ at Sipgate.com. If you're phone is not listed under device configuration just match the other Nokia settings to your phone. They look a little different but the configurations are the same. I will be submitting my screen shots to help them out with the 5530 soon, since my Sip is now working........Thanks to you!!!
Hope it works out for you too.