Well let's follow the money.
When Mitel brought Inter-tel, Intertel was the 6 th largest manufacturer of PBX's, Mitel was # 8 (I think) . That made Mitel # 4', 1 Avaya, 2 Nortel, 3 NEC, 4 Mitel.
That was then, this is now.......
Avaya brought Nortel made them a big #1, Cisco is now # 2, Mitel # 3, before Polycom.
Mitel is the 3rd largest manufacuter of Hosted (SIP) phones. Polycom # 1 by far, Yealink # 2.
Now Mitel has Poylcom, there may no longer be a # 2
Hosted is the future of PBX's (I think), Mitel is or will be soon, the Big Boy on the block.
Of course most of this is my opion base on many years of being part of this industry.