You and JB are right on the money here!(+5) Thought I'd just add a reference;
How to determine IM&P and CUCM compatibility??
Starting with releases 9.x there is no longer a mix and match between CUCM and IM&P, they need to be aligned in the same major release.
For the most part, CUCM and IM&P need to be on the same X.Y level, for example:
- CUCM 184.108.40.20600-11
- IM&P 220.127.116.11900-1
They both match at the 9.1 level and are compatible, even if they are not in the same MR, as long as they keep in 9.1, they will be compatible.
At this time, the only release that breaks the rule is 10.5(2):
IM and Presence Service and Cisco Unified Communications Manager Upgrade to 10.5(2)
IM and Presence Service Release 10.5(2) is compatible only with Cisco Unified Communications Manager Release 10.5(2).
Which as you can see, needs to match at the X.Y(Z) level, always make sure to review the Release Notes for the product before upgrading or installing to make take into account any special considerations like the above.