Since Textline is built for communications with existing customers, rather than for SMS marketing campaigns, we do not require opt-ins. However, if you're concerned, you have a few options.
If you'd like, you can use Textline for largely inbound support, never initiating a text to a customer who isn't already in your Address Book. This would ensure that you are not texting someone who you know does not want to be texted.
If you'd like more explicit opt-in, the Textline Address Book does have notes, tags, and custom fields which can be used to keep track of this. You could also use an online form in combination with our Zapier integration or API to only add contacts who request SMS communication to your Textline Address Book.
We also fully support industry standard opt out messages like STOP, UNSUBSCRIBE, CANCEL, etc, and don't allow messages to be sent to customers who've opted out using one of these commands. Many of our users who initiate texts to their customers will simply add "Reply STOP to opt out" to the end of their first message, to make sure the customer knows they have that option.