Peter, if you connect a phone to the same line*, and pick it up after TiVo has picked up the line, you can hear it dial, and if it gets an engaged tone (or something else). Also, if it connects, you can hear the initial negotiation, before the "real" data starts (though, I might be clouded by years of troubleshooting fax systems on this point
). If it does get this far, you probably no longer have a problem, and can put the phone down (very carefully/slowly/gently!).
I can tell when my TiVo has picked up the line, because I can see it happening on the screen of my answerphone, followed by the number, as it dials. Note, this happens, quite a while after TiVo reports "phone in use", on the screen.
HTH!
Matt.
* - you might have to turn "Phone Control" off in the dial menu for this to work (not to be confused with detect Dial Tone).