Dev Phone throws an error on teardown if a fallback URL is set on the number

[ayyrickay]

Twilio does not allow you to set a URL to empty if there's already a fallbackURL configured. This causes dev phone teardown to error out if somebody configures the dev phone with a number that has a FallbackURL. We should consider replacing the URL with the trial webhook, instead of making it empty (this might also be a better UX anyway, because it gives people a clearer idea of what was going on.)