My phone rings but I can't hear anything when I answer the call

I have an iPhone 7, never had any issues, haven't touched any of the sound settings, but for some reason I can no longer hear the person on the other end of the line when I answer a call and it doesn't give me an option to switch to speaker mode.

For context, my ringtone plays when the call is incoming and I still get sound while watching videos, so the sounder definitely isn't broken, and I'm definitely not in Silent, Airplane or Do Not Disturb modes either. I've also tried changing the volume levels when on a call, on top of downloading the latest iOS update in case something was out of date, and then rebooting the phone while holding down the "volume down" button as well, but nothing seems to work.

Any help would be much appreciated!