From what I can see you can use JavaScript to accomplish this (actually it would be the only way).
Add this code in:
if( /Android|webOS|iPhone|iPad|iPod|BlackBerry|IEMobile|Opera Mini/i.test(navigator.userAgent) ) {
// some code..
}
Inside the if statement I would place an HTTP request to
Twilio. From there you will have the information of the user's mobile device.
Alternatively, you could make use of a PHP form and leave it to the user.
Good luck!