Do the following:

*FIRST put back intercom preference - press PROGRAM button- then dial 99 - then press INTERCOM button - then press STORE (auto-dial/store) button.

*To clear message (#252) lift handset OR press speakerphone (SP-PHONE) button- dial 700- then dial the extension number of THAT phone.

*Now set that phones forwarding to VM (busy/no answer) by pressing speakerphone- forward button (FWD/DND)- then dial 5 277 (vm ext.) -and press speakphone button (SP-PHONE) again.
(if you don't want 'busy/no answer' which is what the 5 here represents -you have these options: ALL CALLS=2 / BUSY =3 / NO ANSWER=4 / AND BUSY/NO ANSWER..once again..=5)
Your message button should go right to vm once that is done.

Let us know how things turned out.