Communication in the modern day revolves and almost depends on mobile telephones. DO NOT GO THROUGH THEIR PHONE... this shows a sign of distrust, and if they aren't cheating... makes you look like an insecure and now, non-trustworthy girlfriend.... How would you feel if your S.O. went snooping through your phone?
They might also be going out then usual. Ask friends or relatives.
assuming you are his girlfriend, i hope you spend enough time with him to notice a change in behavior, or more time that he isn't available for you. giving you less attention and paying more attention to other girls might be a hint if the phone option isn't there.