The bigger problem is that you don't trust him.. :/
My question is: Why does he need to turn off his cell phone?
If he normally doesn't turn his cell phone off he's probably depressed and overwhelmed about something. Try to become more actively involved in his life. Put some verbs in your sentences.
I know several people for work, for school, that turn off their cell phones and forget to turn them back on. I wouldn't sweat it unless there is some other concern you have.
May not have paid the bill?may have dropped it in water?
could be any of many answers!
Not if he works IT for a living! He might just be making sure he doesn't get a support call when he's not supposed to! I say this because it is exactly what I do!
As for the rest... the other posters are pretty much on the money with advice. Sounds like you already suspect him... ask him straight out... while there are some men who can lie convincingly they are pretty rare and your 'women's intuition' should sound an alarm if he isn't straight with you on the answer.
Apparently it's your Communication that is turned off! Talk to him! lack of communication can lead to doubts, misunderstandings, fears, which will lead to lack of trust!
Drix is right, it's clear because of your suggestion that he is cheating suggests you already assume he is cheating. You think he is cheating. Here is what you do. A five step process to help you on all your little relationships.
1) Find your balls if your a man; for a woman just find your courage
2) Figure out what you want to tell him (for you its that you think he is cheating on you)
3) Tell him how you feel
4) Ask him what he thinks/how he feels
5) Trust him (and if you think he is playing games and lying, then there is really no point for you to be with him)