As per https://mms.10gen.com/help/backup/tutorial/register-for-backup-service/, a phone number is required for two-factor authentication. Please add that for users outside the US they may have to prefix their country code with a '011'. So for example, for Australia with a country code of +61, the user may need to enter 01161 followed by their number.

This information needs to be repeated here as well:
https://mms.10gen.com/help/backup/tutorial/