I can only have a conversation while I have speakerphone on my iPhone

Try a forced restart then, press and quickly release the volume up button then quickly press and release the volume down button then quickly press and hold the side button, ignore the power off slider and wait for the Apple logo, then let go of the button. Then try a phone call.

Can you use Hey Siri and have her respond?

If you are already updated to iOS 15.5 then your next step would be to click on the Get Support link at the top right of this page to start the service process.