If your 10 DLC campaign failed because of the requirement for "Opt-in language is required on the website if used to collect mobile numbers," it means that you need to ensure that whenever you collect any mobile numbers on your public website, you obtain the user's consent to potentially send them text messages.
​

Even if the mobile number you collect on your website will never actually be used for messaging, carriers still require this consent.
​

We recommend adding a statement like the following to your website: "By providing your phone number, you agree to receive text messages from [COMPANY NAME]. Message and data rates may apply. Message frequency varies." This way, users are aware that providing their phone number implies consent for receiving text messages from your company.

Another option would be to remove the MANDATORY requirement of a phone number on your form if a phone number is not required.