Just to be clear:
On SOME phones, an intercom call not answered will go to voicemail and you can leave a message.
On SOME phones, a DID call not answered will go to voicemail and you can leave a message.
When the call goes to voicemail, the caller has the option of leaving a message or being forwarded to a cell phone.
OR
Someone pushes a preset forwarding button on the phone and the call goes to a users cell phone INSTEAD of to voicemail?
This is all a bit confusing as we are not on-site and didn't watch you program.