This really old post gives a clue on what the structure is for their firmware numbering.

http://forum.yealink.com/forum/showthread.php?tid=1661

The template of Firmware name of Yealink phone is X1.X2.X3.X4

(1) X1 means the Phone Model

X1- Phone Model

T4 Series
35 - T48G
28 - T46G
29 - T42G
36 - T41P

(2) X2 means the Major version number

X2-Major version number
61 - V61
70 - V70
71 - V71
72 - V72

(3) X3 means the Custom or Neutral version

0 or 1 means Neutral version
Non 0 or 1 means Custom version

(4) X4 means the Minor version
Yealink will release some minor versions after solving some bugs.

I remember sitting with a Broadcloud rep 18 months ago and they were worried about how SFB on Office 365 would affect their business. Still find it amusing that we took it as a serious threat.

I still have a lot of hope for FusionPBX and some other open source products as well.