To send a greeting card as a text message, enter the SMS to an email gateway specific to the recipient's phone number and cell service provider when the greeting card website prompts you to enter a recipient email address. Major cell carriers all offer gateway email addresses for phone numbers.
Consider the example of sending a greeting card to your friend who's phone number is 999-555-1212, and whose cell carrier is T-Mobile. As of 2015, the email gateway for T-Mobile is firstname.lastname@example.org. If you're making a greeting card for this person, you would enter email@example.com. It's important to enter all 10 digits of the phone number.
Other providers have similar ways to get email addresses into the SMS system. For AT&T, the gateway is firstname.lastname@example.org; Verizon's is email@example.com; Sprint's is firstname.lastname@example.org or email@example.com as of 2015.
The greeting card comes to the recipient's inbox in the form of a link, and the recipient clicks on the link to access the card. The recipient's ability to see and hear all of the features of the card vary with the limits of the Internet service on his or her phone. In most cases, people with 4G or LTE service can see and hear the cards as well as use their own Internet service to send a card back.