Am I wrong in guessing this is a Twilio-powered service?

If so, this is a brilliant pricing model for adding value on top of someone else's API. One-time App store pricing of $1.99 gets $1.39 to use on purchasing a short-duration Twilio line and then in-app purchases can be used to refill extra numbers/minutes, all presumably with enough margin to make money on top of the basic "reserve and use a Twilio number" functionality.

Just checked and Twilio phone numbers are $1/mo. and 1-2 cents a minute for calls. There's definitely headroom to make money here.