asterisk users - Apr 2013 - To enhance the voice quality of the SIP trunk

Hello;

I have a SIP trunk with a service provider, the caller from landline or mobile
is hearing us very well, but the agent who is sitting on the handset is not
hearing well, the voice at the agent is not crystal (like he is talking from
well or far deep place). Although the IP Phones are cisco 7942G and the used
codec is g711ulaw (actually it gave better quality than g711alaw).

If we increase the voice volume from the Cisco handset, the voice is becoming
higher but more distortion, it is missing for the sharpness (clearance), we are
hearing it like he is talking from well (from far and deep place).

I am trying to fix this .. I requested the provider to check if the problem from
his telephony card, but it is not doing any thing.

What parameters can help me to fix this? trustrip, insecure, jitter, .. etc? Any
of this can help?

Regards
Bilal