Q: where do i get the phone number from

The iPad is not a phone and therefore doesn;t have a cellular number. The SIM is used only for data connectivity on the operator network.

Adding to Alfred's correct response:

Perhaps you are asking because you see a phone number in the AT&T screen in the HT4157 article you attached.

That phone number is shown assuming the person chose "Add to Existing AT&T Account" above - in other words, in that example, the person has an AT&T cellular phone and is simply adding the iPad to this existing account.

If that is not your situation, choose "Set Up New Account" and you will not be asked for a phone number.

Here is where to find the numbers:

http://support.apple.com/kb/HT4061