In the example of the agreement request in this spec https://mobilepaydev.github.io/MobilePay-Subscriptions/#agreements there is a mobile_phone_number.
Mobile_phone_number is not required, both for agreements and one-off. Neither for agreement creation, or one-off creation. If you choose to add it, the phone number will be prefilled on our landing page. So that the user will not have to type in the phone number here:
So it is slightly more convenient for the user, if you add mobile_phone_number.