You might want to look into perfmon
arr, looks like it indeed. Would there be any reference list of all those counters?
Aha, found it, it seems there's a seperate forum for Serviceability XML ;-)
So for the counters, looks like just using RTMT Performance as a reference and take that directory structure. For registered phones (per server) it should look like this:
/<server>/Cisco CallManager/RegisteredHardwarePhones
Anyways, gonna try it out, all tips and tricks still welcome of course...