Mah437,
We have been trialling CyberData Corporation devices, both the SIP Speaker V2 VoIP Paging RAL 9002 RoHS (this is a ceiling tile speaker) and the VoIP V3 Paging server (a SIP gateway device to analogue audio).
Both are unsupported obviously by Mitel, however I can confirm that both work well as a SIP extension.
So far all our tests have been really good, happy with the audio quality and the volume. The ceiling tile speaker we have is 802.3af POE and has a small internal amplifier that allows you to connect another analogue speaker.
We run the paging server into a 100v amp and run through a PA in our main admin building. This works very well also, but this is only as good as your PA.
As a side note, some of the ceiling tile speakers can be used as a two way intercom.
Regards
Luke