You're not working overtime because you are not working your expected 9to5 duties. Overtime would be if you were actually sitting in front of your computer act

You're not working overtime because you are not working your expected 9to5 duties. Overtime would be if you were actually sitting in front of your computer actively working on your project (coding, answering emails, bugs, feature requests, etc). Just being available counts as a remunerable activity but I don't think you'd be able to convince anyone that it counts as actual overtime duties like you would if you were actually overtime. It's "doing something" more than it is "doing nothing" but it's not as involved as actually "doing work" like you normally would. Hence, you are being paid for it, but it's not your full rate.