Source: jonsi.com
This form acknowledges that the customer may be reluctant to provide a phone number, and provides an explanation for the requirement.
An assurance that it wouldn’t be used for other purposes might be good, but then the message starts to get rather long. And perhaps it’s adequately implied.
One could argue that having the information displayed by default, instead of requiring a specific action on the part of the customer, would be better.
(Good album, by the way.)