It must be said that Twitter/voicemail mashup Pockets has great potential for spammy misuse. Used properly, though, it's a pretty novel way to send a voicemail message to someone you know through Twitter but don't have a phone number for.
Pockets signs you in and authenticates itself with your Twitter account, then asks for your phone number and a Twitter username to send voicemail to. Hit "Send Voicemail" and the app calls you to record a message. Hang up, and Pockets posts an @whoever reply to your recipient, asking them to likewise authenticate their username and then hear the message.
Like we said up top, this would be really annoying, if used as a way around email barriers to push a spam-like message through. But blocking Pockets on Twitter wouldn't be all that hard if you had to, and the app does offer potential for actual, useful purposes. Here's a quick video demo of how it works:
Pockets is free to use, requires a Twitter account and phone number.