Strange issue on transferring call

[Temperor]

Hi,

I'm new to Shoretel phone system. Our company is using ShoreTel 6.1 system. Here is an issue I found recently. I hope someone can help me.

Example:

A customer names John. He calls to our company. Aleasha picks up the call, and John is looking for Bob. So Aleasha transfers the call to Bob's extension. Soon as she transferred the call. She realized Bob is sick today. So Aleasha picks up the call again, and tell John that Bob is sick. After a few seconds, all the sudden Aleasha lost the call, and John somehow got transferred to Bob's voicemail.

Anyone has any idea why the call got transferred?