Trouble using two phone numbers to send messages

Since upgrading to iOS 6 people to whom I send text messages are getting in touch to ask if I've changed my phone number. I haven't. but because iCloud is running on both my iPhone and my iPad, for some reason texts sent from my iPhone appear as being sent from my iPad's number. I go into settings on my iPhone and then > Send and receive, and the iPad phone number is ticked and greyed out, so i can't delete it.

Any suggestions? All I want is for all my messages to be sent from my iPhone number.