TTY calls to my business are rare, but still exist. Really, it depends on your clients -- you're asking a group of tech-savvy people here online, most of us probably abandoned the TTY ages ago.
Personally, the more ways you have of people contacting you, the better. A toll free TTY line, combined with email, and maybe instant messaging, would be accessible to most people. Now, work on getting that video access...
The majority of deaf people do NOT have cell phones. A good number of them do, but it's not everywhere yet. Close, but not everywhere. Again, depends on your client makeup -- tech savvy, or rural and tech phobic?
