Thanks to my buddy John de Breij from Mitel Netherlands. Mitel is now listed on the interoperability program site of Microsoft! So Mitel is now fully qualified IP PBX vendor.
Qualified IP-PBXs for Microsoft Office Communicator 2007
Listed below are IP-PBX and firmware combinations that have been independently qualified viatekVizion labs or at Microsoft to meet qualification requirements. Please contact the vendor for more information on these products. Other supported products may also be listed by request of the vendor as the same firmware is used in these products.
More information see