Try reprogramming your phone using *228 and then choosing option 1. This will reprogram it with the proper settings for your phone number and make sure all of the network information is entered properly.
If that doesn't solve the problem, you will need to call Verizon. While you are talking with them, you should ask them to move you to a texting plan that covers your overage and back-date it to the beginning of your billing cycle. This will keep you from paying those nasty overage rates.