Although you can send an SMS marketing campaign using the phone numbers of your customers directly, it's recommend to important your Contacts into Wix. This way, you'll be able to send future SMS campaign a lot easier.

Here is how you can add your contacts in Wix Contacts: https://support.wix.com/en/article/wix-contacts-importing-your-contacts-into-your-contact-list

Once you do this, the app will be able to detect your newly added contacts. Please refresh the page to see your newly added contacts.