This is one of the biggest headaches I have as a developer of two mobile apps that require number verification. But the headache isn't because the twilio/nexmo/clickatell/etc APIs are too complex, or because I don't want to deal with adding a single verification code column to a user record in the DB schema, but because the delivery success rates for the SMS gateways I've been using are quite bad.
Most of my time is spent trying to figure out which mobile networks which SMS gateways will deliver best to, and then directing the correct verification traffic through those gateways accordingly. Which is a nightmare, in part because the successful delivery rates for different areas and networks constantly changes.
If this is the problem that Prove is solving, I think emphasizing that (or at least mentioning it somewhere) on the website is important. I would be an eager customer.