They can check this 12 hours rule hassle free by just clicking on your staff number and pair this with your ID 90 used ticket. If you are a commuter this means basically that you HAVE to come back 24 hours before the flight and go to your apartment or hotel room. So in turn this explicitly means a shortening of your stay back home. Combine this with a typical 2115 UTC reporting for duty trip on your OFF day and 'voila' you have to come 30 before your next trip. (unless you are planning to get really tired by coming 12 hours before the flight and stay at the crew lounge LOL)
The 24hr telephone contactable requirement is not really fiction even if its on your OFF day just be weary about that. If and for the time you are needed they will keep quite, this can turn ugly very quickly.