Good question, ww.
I was put off by the thought that I'd have to make up a number beginning with 07, followed by nine random digits. Another question which occurs to me is, if the Hushed app generates a number, is that number then withdrawn from the millions of numbers yet to be allocated to an actual phone?
I guess it must do other the scenario you outlined of a non-punting mobile phone-owner getting a number that Hushed is using on, say, my phone would still exist.