The theory behind 2FA is that 2 things have to be combined to authenticate an end-user's identity. Something the customer knows, for example, a password must be combined with something they have, for example, a mobile phone. The overwhelming majority of people these days have a mobile phone that can receive SMS and bearing in mind that mobile phones are rarely more than an arm’s length away, and they are a highly effective way to reach people. Sending a single use, time sensitive text message to confirm a customer’s identity could be the difference between someone’s account being compromised and remaining safe.
Most people have a mobile phone that can receive a dropped call. However, due to user experience flash call is only recommended by Sinch to be used on Android handsets. Still in many parts of the world >70% of handsets are Android, so often if using Android you can attempt flash call with automatic intercept, but then fallback to SMS. For iOS devices it is recommended that you use SMS verification.