Please Help! Voicemail Problems

I have had problems with voicemail being cut off approximately around 31 seconds to 47 seconds. It is usually around the 31 second time. This only happens when calls are coming thru our vonage buisness lines. If a caller dials in thru one of our pot’s lines it does not cut them off. This is has been an ongoing problem for a while that I cannot figure out. I’ve messed with all of the settings in voicemail.conf file relating to length of messages, etc… Any help would be greatly appreciated on this.