If you want to use native Zoom registration pages to collect phone numbers for SMS messages, you need to add explicit consent. 

Here's how:

For Zoom Webinars

For Zoom webinars, here's how to add an SMS consent checkbox:

After creating your webinar, you can edit your registration page by going to 

Invitations >

Scroll down to "Registration Settings" and click 'edit'

Then you can add a "custom question" 

For Zoom Meetings

Zoom meetings don't have a checkbox option, but you can create a dropdown consent by creating a dropdown consent.

After creating your meeting, go to "Registration" > And under Registration Options click 'Edit'

Then on the bottom of the popup, click "Add Question" and add a multiple-choice question for your SMS consent.

Here's an example of what it could look like when done: