Phone verification is a great way to ensure that your users are genuine. It is an increasingly common practice, yet every one has to re-invent the wheel to get it working. Having used it for 2 of our apps, we decided to make it available to general public as a service.
This service uses text messaging to send verification code. So, it's more suited to mobile phones. We haven't needed more than that. However, if you'd like to run verifications via calling, do let us know, and we can add that feature.