Section D1-01 of the CTIA Compliance Assurance Solution Mobile Commerce Compliance Handbook states:
“Handset possession is implicitly confirmed when user opts in from a mobile handset: all other opt-ins for recurring services must include an additional step to confirm handset possession. Service providers may satisfy this requirement by sending an abbreviated op-in MT message soliciting an MO response from the user. This message must include enough information to identify the program.”
A consumer verbally telling you their phone number or entering their phone number on a form does not confirm that the phone number is actually theirs, therefore the need to get an opt-in on the first attempt to text them. There are also other requirements for the language in the first attempt. If the consumer initiates the text from their phone than ownership is implicitly confirmed. Even in this case best practices recommendations are to gather permission on the first text response. The links below give some further insights.
http://wmcglobal.com/media/ctia-mobile-commerce-compliance-handbook-v-1-3.pdf
http://www.ctia.org/docs/default-source/default-document-library/industry-best-practices.pdf?sfvrsn=0