Been doing this since the early days of my PPL after CENSAR called me up angrily telling me off
When I lodge my flight plan I set 3 alarms into my phone and put my phone on silent. (yeah I'm paranoid now). I always turn my phone on loud when I get out of a plane - habbit so it works for me. If I'm still in the air and my sartime is approaching my phone simply begins to vibrate 30mins early in my pocket. A quick vibration = sms/call which I ignore. If it continues for ages I take a look at it if possible and reminds me its sartime.
I remember my old nokias you could actually turn off with an alarm set and even with the phone turned off - the alarm would turn the phone on and start buzzing... have not come across a phone with that ability in years tho