Take a look at FreeSwitch, it can provide a bunch of what you need, and others are using it as an SBC also. Here is what need:

- Voice and video support - Proxy - B2BUA - NAT Traversal - SIP - CDR

Does ALL of these....

These things would be nice: - H.323 support

yup, mod_opal

- Transcoding

yup, you may need to purchase some codecs ( from 3rd parties )

- Simplified Management

guess this is subjective.. but little in the way of GUI's for it. config files are easy though What is not needed:

- Auto Attendant

- Conferencing

Has VERY Good conferencing for what its worth

- Stateful fail-over

yea it dosnt have this, which id love personally. Id seriously suggest you look at FreeSwitch its got a great community, and its very well designed. out of the box the default config is more PBX like, but Freeswitch is a platform for you to build upon. you just need to learn to configure it how you want. Feel free to email me any QN's -- Sincerely Jay