u know voice traffic is RTP traffic and u have to treat it as such,
try the following:
-check your bandwidth capacity against the No. of voice ports u have for calls.
-is your RTP traffics compressed at both ends? if No, do that by enabling cRTP.
-with your codec, consider the payload for the codec used, as this might affect the bandwidth comsumption
Your problem is simply bandwidth management/comsumption..
-Lastly, check all cables
do email me firstname.lastname@example.org if that does not help