Firstly, your question would be better placed in the TelePresence section of the forums where these things are more actively discussed than in the Video Over IP section you have posted to - I'd suggest you move your post.
Secondly, TC5.1.4 is very old and subject to numerous interoperability and security vulnerabilities. It is highly recommended that you update to a recent release (ie TC7.2.0). If you have an active service contract, you can download the software from the Cisco web site. If you don't have an active service contract, you should be able to get an upgrade through the TAC as per the "Customers Without Service Contracts" section in this security advisory -> Cisco-SA-20140605-OpenSSL.
Thirdly, H.323 packet inspection should be DISABLED. Having it enabled will often cause you issues.
Fourthly, there are lots of topics in these forums about using an SX20 as a standalone device and NAT in the TelePresence section - have a quick read through some of these to see if they're any help.
Finally, if none of the above helps, some config snapshots of the relevant parts of the SX20 and ASA configs may help us to see how you have things configured and what you're missing.
Wayne
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Wayne
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Please remember to rate responses and to mark your question as answered if appropriate.
Wayne
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