Here's how to force your iPhone to use your Bluetooth headset when answering incoming calls:
- Tap Settings
- Tap Accessibility
- Tap Call Audio Routing
- Choose Headset
Change Call Audio Routing to Headset to cut down on lost headset connections
The default setting for Call Audio Routing lets your iPhone automatically choose where to send your calls, either the built-in speaker or a headset. Changing the setting to Headset forces your iPhone to look for your Bluetooth headset as its first option, which means your headset controls for answering calls should work, too.
If you leave home without your headset, calls will fall back to your iPhone's built-in speaker so you don't have to wonder why you answer calls but don't hear anything.
Not every Bluetooth headset works perfectly with every smartphone, so there's a chance you'll still have to deal with phantom headset syndrome. That said, everyone I've suggested this to has been pleasantly surprised that their headset now works like a charm.