Make sure that you have set your echo cancellation parameters on your 5400:
For example:
voice-port 1/1:D
input gain 2
echo-cancel coverage 32 (This is the ECHO CANCEL parameter I was talking about)
cptone GB
timeouts initial 120
timeouts interdigit 2
timeouts call-disconnect infinity
timeouts ringing 60
description TRUNK#1
bearer-cap Speech