Your prostition business may well be exclusively in-state. All of the phone calls that your customers make to you may well be in-state calls. But you can still be convicted of violating the federal Travel Act because your use of a telephone is a "facility" of interstate commerce. So holds Judge Clifton.
Pretty expansive view of the Interstate Commerce Clause, eh?
Prostitues in (as here) Montana, or other relatively isolated states (e.g., Alaksa and Hawaii), beware.