Which phone number is being requested when setting up an AT

I had an AT&T data plan for an iPad 2 before leaving the USA on assignment 10 months ago. This was automatically renewed monthly, and the phone number was assigned by AT&T. I am back in the USA for 4 days and tried to purchase their daily $4.99 plan through the iPad. I did not want something that renews automatically, because I will soon be away again. AT&T requests more information now than they did when the iPad 2 came out, among other things, a phone number. I was unable to find out for AT&T what phone number they are requesting... for billing, or what? It doesn't seem reasonable that it would be the phone number of the iPad itself, since they apparently deassigned the number after I dropped the plan... and ir apparently has no phone number at this time... but I don't want to sign up for a data plan for the wrong number, either. Thanks.