What versions of software are you running on your endpoints? We've seen many issues previously on old versions of the VSX software. Upgrading to at least version 9 resolved the majority of them.
The other workaround is you can set a capset filter on your SX20 to restrict it to H.263, but that's not recommended as a long term solution, but just one to investigate any other connectivity (firewall/NAT/call control) issues. I gave an example of doing that in another thread: cisco-sx20-not-working-certain-vsx-polycom-endpoints to fix the Polycom issues. As mentioned in there, we have version 9.0.6.2 (from September 2011) installed on all the VSXes we use and this works well with our Cisco fleet.
Note: Your question would be better placed in the TelePresence section of the forums where these devices are more actively discussed than in the Video Over IP section you've posted in - I'd suggest, if the above hasn't answered your query, that you move your discussion over there.
Wayne
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Wayne
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