Calls going to the PBX should NOT bounce back into Vocera when the line rings busy. At the very least, the user should be notified that the line is busy and the call is being sent back to Vocera. Currently, if the PBX line is busy the call reverts to leaving a message for the group. The caller may not know that they are leaving a message in Vocera vs the PBX voicemail.
The option to disable group voicemail (available as of 5.3) should cover this problem. Since Vocera is designed to route the call as quickly as possible the design is intentional to try to find a logical spot for the caller to land vs. them ending at a dead-end. Understood that a Vocera voicemail box might not always be the most appropriate place to land.