Hello!
Mobile phone off or flight mode?
On. You never know if you need it, it might become your emergency radio. It works nicely at typical altitudes at which SEPs are operated and does not seriously interfere with your onboard com and nav units. If you turn it on only when you need it, you might lose precious time while the phone attempts to connect to the network.
Make sure that important numbers are stored on your phone like those of the most often visited aerodromes and AIS and suchlikes.
There is a famous case of a Piper Malibu that flew on an IFR flight from Poland to my homebase a few years ago. He lost all electrics in IMC shortly after takeoff and continued his flight VFR on top from there on. From the German border onwards he was escorted by to F4s. Throgh his mobile phone he contacted the tower of "our" airport who in turn informed the military about his plan to return home. They even got someone familiar with the Malibu on the phone for him, who talked him through the checklist for emergency extension of the landing gear. This case is now taught in CRM seminars as an example for good CRM because he made the very best use of all available resources!
Happy landings,
max
Ah, and one more thing: We are all humans and make mistakes. To prevent these mistakes from having consequences, a very smart person once invented a thing called "checklist". Don't be afraid to use it. Always. (Then things like those described by BackPacker will not happen...). Don't be ashamed to use it, "we professionals" always do. Even on the sixth sector of a flying day. Every checklist. Every checklist item. Always. And you'd be surprised how often the chcklist catches forgotten items. Even more of them on the sixth sector of the day than on the first!