Don’t forget that not all valid phone numbers can handle SMS messages, land lines for example. That said, SMS verification will likely work for nearly everyone these days. Your WP server cannot send SMS messages unless you install additional server software. The easiest way to send SMS is via a service that offers an API your server can interact with to send messages. Some of these plugins should help you integrate with such a service.
Alternately, there are services that will directly confirm whether a phone number is valid or not without the need to reply to a SMS. Search the ‘net for “Phone Validator”. I’m not sure how reliable these are. Probably only worth considering if many of your customers might only have land lines, such as the elderly.
