There is a bit of overhead in ensuring everything Unity needs (RDNIS, MWI, transfers, etc) gets handled smoothly between SIP and QSIG. The EGW can facilitate this. At this time it is the only alternative to using a CCM or Cisco Sip proxy. The two links below should fill in the details for you...
http://www.cisco.com/iamegw/planning/planning_sol_unity.htm
http://www.cisco.com/en/US/products/sw/voicesw/ps2237/prod_configuration_guide09186a008021f6ca.html
Depending on the PBX, you might also bypass the QSIG trunk and have dedicated lines going to Unity. This would then use Dialogic voice boards or Intel's PIMG (a specialized type of SIP gateway). I'm guessing you've already ruled this out but wanted to throw it out there to be sure.
Regards,
Eric
