It doesn't matter from a "business sense." You cannot force anyone to only use any one relay service. You can develop the device, and make it EASIER to use your service than anyone else's, but you cannot prevent them from using anyone else's.
The FCC has been quite clear on this.
http://hraunfoss.fcc.gov/edocs_public/attachmatch/DA-05-141A1.pdf
Now, Sorenson advocates like to use this line in the above quote:
However, that's not FUNCTIONALLY EQUIVALENT. Hearing people do not have to have 2 different telephones to make calls. Deaf people should not have the expectation to acquire a TTY when they have a restrictive VRS device preventing them from making telephone calls. Therefore, the FCC needs to remedy this right away.