Progressive disclosure is defined by NNG as:
Progressive disclosure defers advanced or rarely used features to a secondary screen, making applications easier to learn and less error-prone.
In plain English, this means showing the user what they need, when they need it. By simplifying the UI, the user can focus on the task they want to achieve. Progressive Disclosure helps reduce user errors and strain.
Whistle helps reduce user strain by not showing the phone number input unless the user decides they need it.