In order to send automations and marketing campaigns over SMS to your customers, you need to make sure that you're collecting your customers' phone numbers.
There are two ways to achieve this in your checkout page:
Allow users to check out with their email address or phone number.
Allow users to enter their shipping phone number.
We recommend setting up both.
1. In the Shopify Admin UI, click Settings > Checkout.
2. (Optional) Allow customers to checkout with their phone number or email address under Customer contact > To check out.
If you require your customers' email addresses for other purposes, you may not want to use this option.
3. Change Form options > Shipping address phone number to Optional or Required, depending on your preferences.
That's it! By completing the above steps, you are collecting phone numbers in your checkout that you can use to send text automations and SMS marketing campaigns.