I had the same problem you describe without the cell phone.
I am poor and my family was poor. My wife and I struggled to get me through college so I could earn a decent living for us. I wanted all my kids to go to college so they could earn a decent living. I knew the only way I could get them through college was to push the grades so they could get in AND get financial aid or a scholarship. And since we are white there are no race based scholarships for us. (i.e. United Negro College Fund)
My #3 son was in your son's position. I felt he was just lazy. So I talked with his teachers. I found out he was not doing his homework or not turning the assignments in. So I got the assignments ahead of time from his teachers and told my son he was to write in a notebook I got for him for each class what the assignments were. (All during this time I'm working 12 hour days.) At first he told me he didn't have any homework. I told him he what his assignment was. Then he told me he already had it done and it was at school ready to turn in. (Either it wasn't or he didn't turn it in.) So I the had his teachers sign his notebooks that he had turned his homework in or not.
When he came home with a history assignment to read, I asked him the questions at the end of the chapter. When he didn'y know the answers, I told him what the answers were. He would ask me how I knew when I found where the answers were in his reading. I told him that I had to go through the same classes he did 25 years ago. He was surprised. (HE he ha ha!) Then when he had an assignment, to answer all of the even numbered questions (He was surprised I knew his assignments), I'd have him answer all the questions. When he claimed, "NOT FAIR", I'd tell him, well, if you are honest with me and tell me your assignments, then you have to do just your assigned work. BUT if I have to tell you the assignments, then you do what I tell you, and tonight that's all of the questions. Your choice. My wife was a math major and did the math assignments with him.
If he had just a reading assignment and I didn't know the answers (often) I'd read his book and then listen to his answers. If he didn't know the answers, then he would have to read the entire chapter over again.
When he complained about not getting to watch TV or play video games, I'd tell him that if he got his homework done he could watch TV or play video games.
I even took a week's vacation and went to class with him. I embarrassed him so much he begged me to go home and he would pay attention in class and bring home his assignments. I warned him that if he didn't, I'd be back in class with him. I smiled all the way home.
My other kids went though the same thing. I told them if they got "A's" then I'd let them study and do home work on their own. If they didn't get "A's" then I had to find out if they weren't capable of "A's" or if the were just not doing their work. Most of them decided to do their work on their own so they wouldn't have to follow dad's study plan.
My #3 went from earning (!) "C's", "D's" and "F's" to earning "A's". He graduated salutitorian (#2 in his class). He received a full ride scholarship for his first 4 years. Then he had to struggle through his post graduate work and finish medical school.
My #3 graduated USC Medical School and is now a Pharmacist in Northern California. Was it worth all of the crying and pouting and "Not Fairs" and working an extra 3 or 4 hours after I'd already put in a 12 hour day? He is the first Dr. in the family. YES! YES!! YES!!! it was more than worth it ! ! ! !
He's earning a very good living and married a wonderful lady as intelligent as he is that keeps him on his toes. They have have 4 kids and are hoping for a 5th.
Would I do it all over again? You betcha! In a New York minute!
BTW, useing dad's study requirements, All my kids did well to the best of their ability. Most, but not all, of them had straight "A's".
Good luck to you and yours.