Why does my iPhone calls ring through speaker instead of headphones?

Re: "my hearing aids are Bluetooth connected as headphones all calls come through the headphones all notifications come through the headphones as set in settings but incoming phone calls ring through the speaker not the headphones"

Maybe you need to use the iPhone's microphone to speak to callers ?

Else: Seems there are at least a couple of options to choose from...

(1) you can manually switch a call to the headphones : See: While on a call on iPhone - Apple Support

Eg: says:

"Adjust the audio during a call

To change the volume, press the volume buttons on the side of iPhone.

Or swipe down on the call banner, then do any of the following:

• Mute: Tap the Mute button.
• Put the call on hold: Touch and hold the Mute button.
• Talk handsfree: Tap the Audio button, then choose an audio destination."

(2) You can: Route and automatically answer calls on iPhone - Apple Support

Eg: says:

"You can automatically direct the audio of phone or FaceTime calls to the iPhone speaker, a Bluetooth® headset, or your hearing device. iPhone can also automatically answer calls after a length of time you set.

1. Go to Settings  > Accessibility > Touch > Call Audio Routing, then choose an audio destination.
2. Tap Auto-Answer Calls, turn on Auto-Answer Calls, then set the duration of time before the call is answered by tapping  or .

During a call, you can switch the audio routing from your hearing aid to the iPhone speaker by removing the hearing aid from your ear."