The V-2924A tip/ring (W/Bl pair) on P7 block is providing intercom dial tone and talk battery. To this phone system, this appears as an outside phone line. An FXO (trunk) port should be used to interface to the V-2924A.
According to documentation found online, the Mitel 5485 provides a standard line level audio output. This output is not compatible with the tip/ring input on the V-2924A.
To test the statements above, I suggest performing two tests.
1) place a buttset or analog phone across the tip/ring input of V-2924A, go off-hook, receive intercom dial tone, enter zone number
2) place a buttset in Monitor position across R/Br pair (pins 34 & 9) of Mitel 5485, page audio should be heard
If talkback and/or multi-zone paging is required, an FXO port will be needed from the phone system.
Here is the information I found online regarding the Mitel 5485:
Here is the install manual for the V-2924A:
Thanks,
Adam