I have seen this a couple of times.
(1) LS trunks not matching impedance. (you don't have)
(2) Jitter/delay on network (probably not)
(3) Not enough echo cancelors (maybe. How many calls are going at same time and how many DSPs do you have?)
(4) PRI is too hot. (This is where I guess it to be in your case.) I'm not sure I understand this, but I had a problem with echo. We worked with the carrier and determined that the PRI was coming in too hot. By adding padding to the PRI smart jack, it resolved it.
Ralph