i use the default sip client on the n810 quite happily with sip service from both teliax and junction networks and my office asterisk server. i like having a little more control than gizmo or skype allows.

call quality is good, a little echo sometimes but much of voip has that depending on your connection. i think the speakerphone of the n810 is actually one of the best i have used for voip.

it also helps with some more restrictive wifi spots if you setup one of your sip accounts to use nat and stun but if tghey block everything but 80, not much yhou can do.