I'll definitely start carrying my phone when I start flying solo, but for now I know that my instructor has his phone and if I take mine, it's just another thing to lose...
I wouldn't fly solo without a phone though, it's another way of making things that bit safer
Although after reading this thread, I might try and find room in my pockets for my phone as well... This may be a stupid question, but if you leave it turned on while flying, does the signal affect the radio/aircraft at all?