The H.323 only limiation is due to the way that the directory is being pushed from TMS. TMS only supports the older legacy management prototcal and global address book (flat file). The GAB was built a very long time ago and was only designed to support H.323/H.320. If you want to support SIP directory entries you need to use the newer management protocal (Dynamic Mode) which uses an H.350 based LDAP directory. Cisco never implemented this so your only option woudl be to use a Polycom Resource Manager for endpoint management.