Hi, @absolutelypeeved. I'm sorry to hear that you have encountered this issue. It's certainly quite unusual.
I'd recommend heading to Settings > Sounds and vibration > Volume, and making sure that the Ringtone slider is at a suitable level. Now, please head to the Phone app > Tap the 3 dots in the top right > Settings > Call alert and ringtone > Vibrate while ringing > On.
The vibration against a flat surface can be quite loud, and can help to alert you to any incoming calls if you're asleep or otherwise occupied.
Aside from this, I'd recommend ensuring that your S10+ is fully-up-date (all apps and the software update).
Here's a helpful thread from the Samsung US Community about making the ringtone sound whilst your phone is connected to headphones:
Please let me know whether this helps.