I know it's printed on the bill...and I know it's in AT&T's system, but I like to keep documentation on virtually everything. I know that those of us with an unlimited data plan are grandfathered in, but since there have been so many questions, and so many concerns, about possible mess-ups, it would probably be a good idea (and will only take a moment of your time), to go the ATT site...Sign in...Go to My Profile...Then click on Customer Service Summary... A PDF file will come up with all of your account data INCLUDING the number on your sim card. You can either print it out, or save it to your hard drive. I have saved it to my "Receipts" folder...just in case. I know it is probably overkill...and I know it is on the bill...but you never know when this MIGHT come in handy.