Twit2Tel.com
Looks like a new company, Twit2Tel, is offering free calls (via computer or mobile phone) via twitter to other registered Twitter users.
How it works:
Sign into Twit2tel.com through your Twitter Account.
-Use the control panel to change your settings.
-Go to Twitter and post the command: @twit2tel call or use the Twit2tel.com Quick Call feature in your control panel
-Wait a few seconds for Twit2tel to call you
-Listen to Sponsor Ad
-Connection
-Talk freely for 4 minutes (if the other user is signed up to Twit2tel.com) or leave a voicemail
-Hang up and repeat
-If you have any problems feel free to call us using Twitter at @twit2tel call twit2tel_direct or 415-763-5943
(borrowed from another great freebie site....I haven't personally tried it)