Should AT&T be held liable for this?

Should AT&T be liable for not informing my father at an earlier time of unauthorized phone services made on my cell phone? My father pays my cell phone as part of a cell phone deal he has with the company. So I have little clue what goes n with the bill. On nov 7 someone stole my cell phone and made several calls from Nov 11 to Dec 1. He canceled it on Dec 10. I didn't know I had to report it lost or stolen and merely thought it was destroyed because I called it several times and got "Were sorry the number cannot be reached." I feel they should have informed him when the first call was made and if they had this whole mess would have been diverted. I mean doesn't a credit card company call you when they notice suspicious purchases? They claim that it is his responsibility to know the activity and trust of who he gives the cell phone to. I still think they should have called him when the first call was made rather than call on Jan 24, two months later and tell him you owe 1000 dollars.