Hello @penny e,

Great question!

Contacts who are identified by Klaviyo should not need to provide their email address again for the SMS consent to be synced to the individual profile. This sort of behavior is explained in our How SMS Affects Profiles Help Center article.

I would also keep in mind some behaviors your customers may take that may deviate from this standard response:

Some scenarios in which a customer's contact information will not merge into one profile, or will merge in a way you may not expect, are listed below:

• If an email subscriber opts-in to SMS with a phone number that does not already exist in Klaviyo in an accepted format, but no email is attached to that action, a new profile is created.
  • Then, if they check out using their email (which already exists as a separate profile) and consents to SMS with that same phone number, the two profiles will not merge automatically.

• If a customer with SMS consent makes a purchase and inputs a different phone number at checkout, that new number will override the information from the previous number on file (if there was one).
  • If you don’t have consent for the new number, but had it for the previous one, it will wipe consent from that profile and replace the contact information with the newly established number. You will then need to regain consent with their new phone number.

For these reasons, although it is not necessary to collect an email field, from my experience, it is best practice sometimes to do so in order to prevent duplicate phone number/SMS only profiles from being created for existing contacts. 

I hope this helps!

David