You can listen to FM radio on a phone without an active data connection, but only if your phone has an activated FM chip, and only with the right app.
How to Unlock FM Radio on Your Smartphone
Wireless Charging Station with Alarm Clock FM Bluetooth Speaker丨RAUGEE — RAUGEE Official Website
Unlock the secret FM tuner in your Android phone - CNET
How To Listen to FM Radio on Android
4 Best FM Transmitter App for Android: FM Radio Apps Without Internet - JoyofAndroid
5 of the Best Radio Apps for iPhone - Make Tech Easier
5 Best Free FM Radio Apps For Android 🔥 ✓
When my Android phone (and its FM radio) proved clearly more useful than my iPhone : Global Nerdy
Radio FM: Music, News & Sports on the App Store
Create your radio's iPhone and Android mobile application - RadioKing