This information is only applicable to Verizon users in the U.S.:
You may experience that text messages or multimedia messages (MMS) that you send are not received by your contacts. You may also notice that you don't receive texts or multimedia messages. To remedy this issue, make sure you have activated your number at Verizon.
To activate or use an existing number on a new BYOD (Bring Your Own Device) phone, make sure to log in to your account on verizonwireless.com → My Verizon → My Device → Activate or Switch Device and follow the on-screen instructions. Alternatively, you can call Verizon customer service to activate your number.