Hi Will,
Few things to make sure of:
'show sccp' to see if they are all showing ACTIVE and registered.
Then check your DSPfarm profiles to see if they allow the codecs you're using. If you're doing G.729 over H323 but your profiles are only for G.711, then you would have a problem, for example.
You need MTP to do fast start, which some voice applications require. As well, it is also needed for a number of supplementary services on CUCM.
hth,
nick