asterisk users - Apr 2006 - Echo in some queues but not others

I have a customer who is reporting very bad echo in only one or two of his
12 call queues.

The phones are cisco 7960 and 7940 running SIP6.3.

No problem with echo when an extension is dialed direct, internally or
externally.

Has anyone seen this before? And has any ideas about how to get rid of it or
what is causing it?

Regards,
Joe