[dredmorbius]

Basically: selling the ability for a company to make phone calls from numerous systems all of which appear to originate at a single, designated, known, and generally callee-approved, number.

Needn't be a walled garden.

Though migrations from PSTN to various alternatives is also fairly likely. Much of the present "social media" / apps space is actually probably a jostling for supremacy / positioning in this regard.