Phonesify provides a plugin for website to verify a user's phone number. This can done either via sending an SMS with a verification code, or making a voice call and reading out the code.
Verifying phone number, especially on mobile apps, is getting more popular. WhatsApp, Viber, Blogger, Facebook.. to name a few. Verifying phone number is also more popular because email verification is not very strong to be used as an identity anymore. However, a trade off is that phone number verification is more costly.
The price of using Phoneify doesn't come cheap. Right now, they have not provide their pricing model, but they claimed to give 50 free verifications, which upon sign up will show a $20 credits. Hence, I estimate it to be 40 cents per verification. If it is 40 cents, it is a very costly verification, considering that SMS is as cheap as 1 cent.