At the following URL
the MCS 7816 is not listed as a viable OCS presence gateway, although it is listed for up to 500 CUPC users supported. I have a very small office that wants the OCS/Presence gateway (under 50 users). Is the 7825 the minimum hardware supported?
MCS-7816-H3 and MCS-7816-I3 are listed as compatible model for CUPS 7.0.x installation.
I don't know why they are not listed in the "Microsoft Office Communicator Interoperability" table. If you have a very small deployment, it should work.