Using the following setup and implementation, we can send SMS notification using Salesforce Flow. In the below implementation, I have used a custom object.

Messaging Channel is needed. It's not a free feature. Digital Engagement Messaging license is needed to get Messaging Channel.
Steps:
1. Create a Messaging Template.
2. Create a Flow.
Note:
1. Messaging channel requires Digital Engagement Licenses.
2. Phone_Number__c is custom Phone data type field on the Custom Object.