does kinda sound like network issues... but "static" is odd. Is this real "static", ie: scratchy audio, you can hear the person... or just white noise, which is actually what decryption errors in the audio stream sound like.
If these calls are going through MBG's, they have call stats in their logs. If you can provide a date/time of a particular event, you can find in the logs (or mitel support can) and see what's going on. This goes for disconnects too.
One thing, if you are prioritizing rtp over signaling and the pipe is full, sets will disconnect if their signaling "heartbeat" messages between the 3300 and the set is delayed for too long.