You may want to inform your contacts that they have the ability to opt-out at any time. Here's how to track and manage those that do so:
Using an SMS gateway
Twilio will automatically unsubscribe your contacts (preventing them from receiving a response) when they send a message containing one of the following keywords: STOP, STOPALL, UNSUBSCRIBE, CANCEL, END, and QUIT. START and YES will opt contacts back in to the messages if and only if they’ve been unsubscribed. HELP and INFO will return a stock message informing the contact that they can use the above keywords to control the delivery of messages. If a contact wants to unsubscribe, they’ll receive the following stock message from Twilio:
The opt-out keyword prevents your contacts from receiving additional messages from your number unless they opt back in. Despite this, they’ll remain in your flow until they expire from it.
Tracking opt-outs from your account page
You can track opt-outs manually within your account page by clicking your Twilio number, then clicking "View Log" in the "ERRORS" panel.
You'll see the number of the contact who has opted out, as well as the error description: "HTTP 400 error: 21610: The message From/To pair violates a blacklist rule." Twilio returns this error if you attempt to send an SMS to a recipient who has previously replied STOP, STOPALL, UNSUBSCRIBE, CANCEL, END, or QUIT to you. You will not be charged for the message attempt. As mentioned prior, to be able to send messages to such recipients, they must first text START or YES to the Twilio number to opt in again.
Other ways to tracks opt-outs
To track opt-outs, you can:
Create a group that represents your unsubscribed contacts.
Create a flow that adds contacts to your “unsubscribed” group and assign it keywords. If you’re using a Twilio phone number, you should create keyword triggers that mimic the keywords STOP, STOPALL, UNSUBSCRIBE, CANCEL, END, and QUIT
Add an Update the contact action that indicates whether a contact has unsubscribed. We’ve created a contact field called “Unsubscribed” that will allow us to see if a contact has opted-out by checking their contact profile. Each time a contact sends one of the keywords that trigger the “Unsubscribe” flow, the “Unsubscribed” contact field will be updated to contain the word “Yes.”
You can even use the Send a message to somebody else and Send an email actions to notify yourself or others when a contact unsubscribes.
Once you’ve completed these steps, you’ll have 4 ways to track opt-outs:
Viewing the contents of your “unsubscribed” group.
Viewing each contact’s “unsubscribed” contact field value.
Sending a text to yourself and/or others when a contact unsubscribes.
Sending an email to yourself and/or others when a contact unsubscribes.
Educating your contacts
Unless you’ve notified them, your contacts aren’t likely to know that they can use certain keywords like “start” or “yes” to re-subscribe to your service. This in mind, it may be helpful to add opt-out and opt-in information to your service during registration. Assuming you’ve collected your contacts’ email addresses, the Send an email action also allows you to follow-up with a contact after they've unsubscribed to ask why they unsubscribed and let them know that they can opt-in using certain keywords.