Alternatively, you could create a calling policy in the Teams Admin Center which enables Busy on Busy, assign it to your colleague, only to this colleague which will give the calleer a busy tone as long as your colleague is busy in meetings, another call or else. If you combine both "if unanswered" and busy on busy it can help. Although there is no setting to only forward a call if the user, your colleague is offline, as far as I know.