So, I had the same problems. Called T-mobile, gave the Apple sim card number and all the people at T-mobile were having major issues with it. "that's not the sim card number".
What I found out is the number printed on the Apple SIM cards is NOT the Sim card number. It's essentially a useless number for activation.
Here's what to do, to get the apple sim card number. Call prepaid activations (if you are going for a prepaid iPad plan) or activations department. Tell them you have an apple iPad that you want to activate with it's apple sim card for a plan. "what's the sim card number?" they will ask. You say, "I don't have it, but I was told you have a tool for deriving the number." Ask them to use their CSN/Sim number tool to get the number. You will give them the CSN number located in Settings / General / About, then they will be able to get the sim card number of that apple sim, and tell it to you, or just setup your account for you.
NOT as simple as it should be, but it works. I hope that my many hours on hold and getting hung up on, and forwarded around, will benefit you all.
