Phone Calls Come to Twitter with @call @username

First off, the barrier to entry is -huge-. Both people need to have accounts on both services. That's four accounts for one call.

Secondly, it's allowing you to activate a service via Twitter that should already be easy to do. If it's hard to make a call on Jajah to another Jajah user, they're doing something wrong.

Third, Twitter isn't a command line. I don't type things into Twitter that I want to do. It's a simple, many-to-many communication channel. This doesn't fit in to the way people use ( or don't use ) Twitter currently.