I don’t have the answer, but why not use Attentive forms and have emails sent back to Klaviyo? (I know that’s possible through the integration). 

Hey @Kelly 

Thank you so much for asking the community a great question. Hope I can help @Mailbox Manny answer this for you!

You do have to ability to add webhooks to a flow, which will allow you to send their phone numbers once they have reached a certain place in a flow. So an example could be you collect their phone number from a Klaviyo signup form and then use that list to trigger a welcome series flow. You can add a webhook at the beginning of the flow that will send the phone number to Attentive. 

The article I linked at the beginning will walk you through how to set it up. However just keep in mind that it can be a little complicated to set up.

Also, @Mailbox Manny is right, if you don't feel comfortable setting up webhooks in your flows, you can use Attentive forms and send profiles into Klaviyo as an easy alternative. Hope this helps!

@stephen.trumble - Do you have to use zapier to create the web hooks for this?