Ultratec Compact/C TTY, is my choice. It includes an adaptor that connects to your BlackBerry, and as long as you enable Phone->Options->TTY on your BlackBerry.
Another option is to use Lormar Logic, and call a TTY directly with your BlackBerry. There are also PC-based "TTY-over-Internet" products (I think Nxi communications or Nextalk or something), so if you use a BlackBerry as a modem for laptop, it's also possible to call a TTY that way. But some of these services are pay-services, although two years of such paid TTY service (directly on BlackBerry or laptop) might be cheaper than the cost of a TTY.