Jili for Tweets lets you receive or make voice calls using your twitter @names and #tags without needing a phone number. Your username is your phone number and #tags lets you have a voice "huddle" with many other twitter users at the same time.
You can control who is allowed to call you, followers, those you follow or anyone. Or create a voice #tag for your followers to join you on a room like a conference call.
Jili also lets you forward your calls from @names to your phone number so customers, followers, fans can be directed without knowing your number.