bad QOS or the non-existence of QOS on the network would be the problem.
it does not matter how the call is routed since the RTP is between endpoints so you can take the GK from the picture.
you would need to look at every single device thru which the RTP travels to verify where the issue is. but just telling by the problem description QOS (or lack of it) is the most likely cause
HTH
javalenc
if this helps, please rate
HTH
java
if this helps, please rate