Having a "dedicated TTY line" is (for most people) just a matter of having a distinctive ring for the TTY (free, or under $5/month) ... it works on a single phone line and simply allows users to identify if the caller is calling the TTY or voice by a single, or double ring. It doesn't impact the voice capabilities of the line at all.
Of course you don't need this service at all if you live alone (or with other Hoh/deaf people who also use TTY - as all calls will be TTY calls (or will require a callback via TTY).
I simply mentioned it in case you happen to live with someone who is hearing - as the distinctive ring it is very very helpful (and often free) in this situation.
