If you just ported your number from another carrier, the porting process might not be completed or has a glitch. Call customer service from another phone and have them verify the port is finished and/or reset your text message feature form their end.
I'm most definitely NOT a VZW employee. If a post answered your question, please mark it as the answer.
Welcome to our family dm123. As a new customer I know there is no fun in having text issues. Ann154 is correct, the porting process may be taking a while to complete. I would also remove all third party messaging applications from your phone, then test out your messaging. Are the problems with specific contacts? Try restoring your phone with iTunes. Keep us posted on your issue.