Why will my iPhone not receive iMessages for my phone number?

I bought an unlocked iPhone and i was texting people through iMessage when I had Wi-Fi. Now I'm getting messages through normal texts only. When I go to the "Send & Receive" option under message settings, my phone number is there but it is gray in color and I can't select it. All the email addresses on the account can be selected but not my phone number. Can anyone help me?

Sign out of imessage, then sign back in again.