Authenticate Cellular Validation Code

I have a simple form where a user types in his phone number and chooses his cellular carrier from a select list. Upon submitting the form, the backend sends a randomly generated validation code to the cellular phone. The user must enter the validation code to authenticate and continue to the next screen. I have attached the simple form and simple html. These must be converted to php. The user first goes to [login to view URL] then goes to [login to view URL] and finally goes to welcome.php. No user can access [login to view URL] unless they have been authenticated by entering in the correct validation code. Every time a user enters a phone number, he will get a different validation code. The user must enter the last validation code generated. The validation code is sent to the phone by sending it to the phone's email address. This should work in the United States, since very phone has an email address. The email address is of the form 10digitphonenumber@carrierdomain, where carrierdomain is contained in the value of the select list. For example, a user enters the phone number 2102223333 and selects Verizon as carrier. The validation code is sent using php mail to 2102223333@vtext.com. All coding should be done with php and mySQL.