I'm with Mike; the trouble lies in the cable run between the door and the Partner.
Neither distance nor polarity is an issue, so it's down to how much time you want to spend troubleshooting the existing cable run.
With all the installation experience you have, I'm sure you know not to rely on a toner alone to locate suitable pairs. Use a multimeter to find the short, ground or whatever it is that's killing the voice path. I'm willing to bet that there's a splice, tramped pair or a breach in the cable jacket somewhere (especially after the way you described the pre-existing wire elsewhere).
