Phone was added as a mandatory contact field starting with Luminate Online release 17.1.  The requirement was implemented to ensure deliverability across all target types.

Click here to see the Luminate Online 17.1 release notes.

Note: phone number is not required for custom targets, but if there are any legislators selected in your Action Alert, then phone number will be required for all targets, even if it's just one legislator among many custom targets.