A little more background would help. It could be several things.
One-way audio to the voicemail application could cause this. After five seconds of silence it may drop the call. Likely a NAT traversal thing. Is there a partial message recorded?
DTMF talk-off is another. Some people's voices have similar frequency characteristics to touch-tones and could falsely trigger a DTMF receiver in the voicemail system. This would seem random, typically the same caller, cut off in mid-word on a vowel sound like "ah" or "oh".
Also possibly a configuration setting for maximum message length that is set way too short. Perhaps it is calibrated in seconds and someone thought it was in minutes.
More detail is needed. Does it happen sometimes or all the time? Is there a partial message recorded? Only to certain extensions?
Audio is set to 8 seconds of silence before dropping call.
Max message length in set at 240 seconds.
This is happening most of the time to this one user. At least she is the only one reporting it. I believe a partial message is left by caller but then is cut off after about 5 seconds.