Is this a new phone or one you've had for a while? Did you recently make any changes to the operating software? If neither of those apply then do the following: Power the phone down, remove the battery, remove the SIM card. Put the SIM card back in, put the battery back in, turn the phone on, text someone to call your number, see if the problem is fixed.
Also, if your telephone number was ported to your phone by Verizon contact customer service and make sure Verizon still owns the phone number. Occasionally an ILEC will receive a request to port a number back if the new owner does not complete ALL of the requisite paper work and requests to initiate the change. This is especially true when an FOC request is missing or incomplete.