To help a customer who hasn't opted into SMS messages but has a phone number, you can use Twilio's Programmable Voice to send voice notifications. This approach enables you to reach customers that are unable to receive text messages, such as those with landlines or no smartphone capabilities. With this method, even if users don't answer the call, visual voicemail will transcribe your message and provide a similar result as an SMS. By leveraging Twilio Functions and Programmable Voice, you can easily implement a system that sends voice notifications to customers who haven't opted into SMS messages, improving their overall customer experience.