Twitter has too many prerequisites for it to be useful in a situation like this; you need a cell phone whose number is not linked to a previous Twitter account, with MMS capabilities and a network that supports it, or a computer with access to the internet, a usable keyboard and a browser/email client. It's actually just short of a miracle he was able to use a cell phone while in jail—most jails will only allow you access to one POTS call, and many times you do not have access to the keypad while you are using it.
There needs to exist a service that will accept incoming vocal transmissions, transcribe them, then post them on Twitter.