I can't text.

Do you have a plan that includes texting? You need to have a plan that includes it. If you are just trying to use iMessage then that requires a wifi connection and sending a message to a person using an iphone. You also need an SMS plan for that as well and it has to be active.

If you need more specific information then we would need to know the error message you are receiving.

Have you contacted your cellular provider to make sure that your account is properly provisioned?

Are you referring to SMS or iMessage (or both)? Do you have SMS provisioned in your cell plan? Have you contacted your carrier?

Call your carrier. When you changed phones, they may have screwed up and canceled texting. Happened to me twice with AT&T.

Most carriers require you to have texting activated. It sounds like they did not activate texts when they switched out your phone.

Once again, CALL YOUR CARRIER to verify your plan. Don't ASSUME anything, particularly from The Dude...

What is the harm in calling your carrier and confirming? Like modular747, I've seen things like this happen when phones are switched on the account.

Not necessarily.

Try this: Settings > General > Reset > Reset Network Settings.