Since we are going to be using the MSISDN as an ID for Loop we need to have access to mozPhoneNumberService to generate the internationalized version of a user phone number as this is the format stored in the server side.

Actually, we also need to expose https://mxr.mozilla.org/mozilla-central/source/dom/phonenumberutils/PhoneNumberUtils.jsm#129 Paul, Jonas, do you think that it is possible to add that function to the API and expose it to privileged apps? FWIW the API is https://mxr.mozilla.org/mozilla-central/source/dom/webidl/PhoneNumberService.webidl

What does the function do, and why is it needed? I.e. how do you expect webpages to use it?

I think this bug is a duplicate / very close to Bug 938265

Actually, we don't need this any more. The MobileID API makes sure that the verified phone number is provided in international format, so as long as Loop uses this API to get the user's phone number for the registration process we are good.