I believe you have to caculate the resources correctly. CCM with MGCP to this router, you should be able configure the resources.
You may have only 6 sessions with 4 calls per session configured.
If you had 24 sessions, there could only be one person on each session, which doesnt make sense.
Conferencing/Transcoding1: Supports up to 50 eight-party G.711 ad-hoc conferencing sessions, or up to 32 eight-party multi-codec (G.711/G.729) ad-hoc conferencing sessions. A maximum of 128 transcoding sessions between G.729a and G.711 codecs, and 96 transcoding sessions between G.729 and G.711 codecs.
( if you max out the router)
http://www.cisco.com/en/US/products/hw/modules/ps5365/products_data_sheet09186a0080191d41.html