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.