Voicemail Mystery

Hi all,

This is a great mystery to me.

We noticed that...
1. When we transfer a call to an extension, the caller is able to leave a voicemail
2. When someone calls us and dials an extension, they would still go to the correct mailbox and hear the recording and a beep. However, after the beep, the phone would just time out in ~20 seconds then hang up. No voicemail was generated.

Does anyone has any insight how this would happen?

Thanks!