Opt-in is the recipient's prior agreement to receive SMS from you, required for marketing messages in most countries and best recorded with the number, the date and the source.
Consent is the legal basis for promotional SMS almost everywhere, and the practical basis for deliverability: people who asked for messages do not report them as spam. The form of consent varies (a ticked box, a keyword sent in, a sign-up form), but the record should always include when and where the number was collected.
Transactional messages do not need marketing consent; they follow from the relationship. Mixing the two, by adding an offer to a receipt, converts the receipt into marketing.
At SMSMeteor: the acceptable use policy requires that lists come from consenting recipients; harvested numbers close routes.