Upgrading to the latest version fixed the issue. However, here is description on how it happened.
For example, if I call extension 215 from inside or outside, it doesn’t really matter. The person on 215 picks up and transfers me to lets say 233, I can hear the ring for predefined amount of time and if no one picks up the call it drops the call. If I call 233 directly it will ring and if person doesn’t pick up it goes to the voicemail.
This happens because the calls go first to a ring group. When calls come through a ring group, the diversions like voicemail are not allowed.
We will analyze it and see what we can do about it.
How would that randomly affect the transfer? The call is being answered by the member of the ring group and then transferred to different extension. I tried multiple times today and test phones and it only failed once.