I've created a DE that's populated via entries on a Smart Capture form a user completes via a link in an email. The form captures the user's submission for 'MobileNumber', passes the 'ContactKey' as a hidden value and has 'US' as the default 'Locale.' An Import Activity in Contact Builder extracts records from this DE and places them in SMS All Contacts. While the activity does import contacts, it's not passing the 'MobileNumber' field (despite being required in the source DE) and somehow is passing an email address despite that field not being in the source DE at all.
I thought maybe this could be happening because Contact Builder recognizes the 'ContactKey' as existing and is only passing the values it knows of, but that's not the case as these test records did not previously exist.
I'm fairly new to MobileConnect and the documentation is sparse at best.