You can invite a call from someone on Twitter without passing them your contact details. Within the Phweet service each user establishes how they’ like to be contacted by either SIP, PhoneGnome, Gizmo5 or FWD.
They also seem to have Flash based soft phone that can invoke the call right in a browser window. In my case Phweet points to a SIP URI hosted by Junction Networks, making the resulting call free for me. It rings through to a Polycom phone on my desk, snom m3 cordless and soft phone when I’m traveling.
The service just launched in a public alpha. I wish them well. I will certainly be giving it a workout.