the mailchimp's standard API endpoint, such as POST /lists/{list_id}/members, require an email_address field, making it unsuitable for adding SMS-only contacts.
Adding SMS-Only Contacts:
In order to add contacts without email addresses, you can import them via a CSV or TXT file. This will allow you to include only phone number for SMS marketing. (https://mailchimp.com/help/set-up-your-sms-marketing-program/) (https://mailchimp.com/help/about-sms-marketing/)
You can follow these steps to import SMS contacts:
create a CSV or TXT file containing the phone numbers of your SMS subscribers. (https://mailchimp.com/solutions/sms-marketing-tools/)
You need to also make sure that you have consent from those contacts to receive sms messages. (https://mailchimp.com/solutions/sms-marketing-tools/)
then you import your contacts using MailChimp's import tool to upload your file and add the SMS contacts to your audience.
Also, before you import sms contact, you need to set up an SMS Marketing program in Mailchimp. (https://mailchimp.com/help/use-send-sms-actions/)
Workaround Using Placeholder Emails:
using placeholder email addresses to bypass the email requirement is not really recommended because it may violate Mailchimp's terms of service. (https://mailchimp.com/developer/transactional/docs/fundamentals)