Regarding packet size g729

I finally got around narrowing the problem i have been having with g729a voice quality for that sometimes it was very choppy
and some times very low and hard to hear. With the adsl connection and single user with licensed g729 i was expecting it to
be much clearer. It seems as though my ADSL modem is playing up. Its Zyxel 660H and looks like it has different packet size for different codec. Could it be the problem with the sound quality? any one has any suggestions on fixing it at the modem if
there is a knownn problem. I googled but wasn’t able find any thing in this direction.

thanks in advance.