Is the software on the Polycom devices up to date? We've experienced issues with Polycoms before, especially when they were running ancient software versions.
Some of the older Polycom stuff also doesn't do H.264 nicely, so you can use a Capset filter on your SX20 to reduce the call to H.263+ or H.263 by disabling H.264 with the following CLI command:
xConfiguration Experimental CapsetFilter: “H.264;H.264RCDO;H.264NIL”
Note: you should generally only use the Capset Filters as a troubleshooting tool and not as a long term solution.
PS - You should also upgrade the software on your SX20 to TC7.2.1 to address the Bash Shellshock security vulnerability, and ISDNLink IL1.1.4 for the same issue.
Wayne
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Wayne
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