Unless the client has built in specific support for the phone protocol extensions, the only thing you’'ll likely see is the presence change when someone is on the phone. Be sure to contact the author of your favorite client and encourage them to add specific support for the other features.
Unfortunately, the protocol is still so new (2 weeks) that no public clients support it yet. As clients do add support, we’'ll keep the list of clients up to date on the clients page.
Would love to see it in GAIM and in Fire (OSX). Would be great if the plugin could be figured to send at least some text information as an IM to the user if a call comes in with the appropriate details. And that this feature could be toggled off if a proper client is being used.