While not common, some phone numbers may have short code texting blocked with their wireless carrier (more often with Sprint/T-Mobile than other carriers). If they do have short code texting blocked, they will not be able to join your list through a short code keyword or website form. If they text your keyword and receive an auto-response that says "Service access denied", they likely have short codes blocked on their wireless plan. They can either contact their wireless carrier and enable short code texting, or you can add them into your account through an Import, You could also have them text in your long code.