SMS is here, but contacts still require an email

This was requested before, but 3.0 changes things: you've shipped native SMS campaigns (many thanks for that!), yet a contact still requires email as the mandatory unique key.

We're running into exactly this right now, migrating a database where a portion of records have only a phone number and no email. Our only options today look like dropping them or creating placeholder emails, and we'd rather avoid fake addresses for deliverability reasons.

So I'm wondering: would it be possible to allow phone to act as the identifier when an email isn't available? Email could still stay the unique key wherever it's present. Thanks!