I can only make 4 simultaneous calls on my MVP800.

The older H.323 firmware is limited to 4 calls. Support for 8 simultaneous calls requires:
- 3.52 firmware.
- Limited to a MultiVoip network
- Limited to a G.723 or G.729 coder
Newer H323 firmware can be downloaded from:
http://www.multitech.com/en_US/SUPPORT/Families/MultiVOIP/firmware