We have a Trixbox server on a dedicated T1 line using SIP trunks. Six Aastra 9133i SIP phones are registered to this server on the LAN. Sometimes while they are using these phones, the external caller's voice will become very distorted (like the call has just gone underwater or very robotic) and the inside users will not be able to understand the external caller. The external callers will have no trouble with distortion or with hearing the inside user while this is going on. This happens randomly at least once a day. However, the recordings of these calls show no sign of distortion. This happens on our Trixbox with our Aastra phones on another site completely as well. Is this something that can be fixed by adjusting the jitter buffer? Are there any suggestions on how to diagnose this issue or fix it?