I don't think you can use your ipad number to receive a txt message. I think that only works for the phone....
Also, for iPad...you can only send and receive messages via imessage...that means the other person my have registered their email address for use with imessage. And you as well. Once this is done, then you can send imessages to/from others via Apple's servers, not via the cellular system.
Now, if you have an iPhone...regular sms txt messages and imessages are combined into a single app....so it gets a bit confusing....when you type into the message box, if you see "iMessage" that means the recipient is going to get an iMessage from you. If you see "txt message" or something similar, that person is going to get a regular txt message from you.
Things are a changing, though. I recent heard that folks will be able to use the iPad for regular txt, but I don't think that has happened yet....