IPhone 4 not receiving calls problem?

I have lately had a problem with my iPhone 4. I've had it since easter, and received calls fine. On Saturday my friend said he tried to ring me, but my phone was switched off (obviously went to answerphone, so he presumed it was off) but my phone was on all day with me, I had signal all day, and my phone never rang! When were together the next day, he rang my phone whilst they were right next to each other, and my phone still didn't ring!! Yes, he has the right number before you ask :p