[HELP] Asterisk IP10s

Hi i have a problem with asterisk and ip10s terminals. I use the 711a law codec,10Mbps lines and ip10s SIP terminals, on the upload the audio is greate but on the download to the ip10s is presend some sort of cutting of the audio. Sometimes i cant hear the last part of the words or even worst case entire words. What can it be wrong I cant find the solution for this problem. I tried to adjut the jitter buffer on the ip10s but it doesn`t help.
Any ideas??