Hello,
From my personal experience with SIP phones, but not Cisco, the traffic 
shaping may influence your voice quality, not echo.
Echo should be canceled by the SIP phone, which usually has a configuration 
option to enable or disable echo cancellation.

Heavy traffic may create drops in the voice link, a missed word or part of 
an word, or force the SIP phones to negociate a lower quality codec.


-- 
Adrian Basescu