No, it is not the phone that controls the call routing, but the call manager.
my guess is the call manager is configured so that if no attendant is available , it will reroute to voicemail.
if not configured using the call manager, there is no "button" to simply reroute the call from voicemail.
-> you will need to check the Call managere settings,
and if necessary reconfigure to NOT use the call manager attendant but forward all calls to a simple IP phone