When Customers opt out of receiving SMSes from a particular phone number you used to communicate with them, a carrier systematically manages their request, and further communication is blocked. This prevents you or them from sending additional SMSes unless the Customer opts-in again to receive SMSes from your company.
Currently, there is no way to generate a report or an exact list of unsubscribed Customers. However, you can use webhooks to capture changes to Customer Profile details, including changes to Customers losing the Mobile tag for their phone number, which indicates an unsubscribe.
The webhook setup works like this:
Create and configure the CUSTOMER_PROFILE/UPDATED webhook to listen to Customer Profile updates.
Retrieve the profile using the GET Customer Profile API.
Check if SMS is OFF for any mobile phone numbers in the Profile.
If the smsPreference is set to OFF for any of their type=MOBILE phone numbers, the Customer does not allow SMS for that number.
The setup above created a notification for every profile update, not just phone number changes. Additionally, you will only be able to tell the current state of a Profile (e.g., the Customer currently doesn't allow SMS, but you won't know that they once allowed SMS).