iPhone 7 incoming calls sound muffled or hollow

I've just bought a new iPhone 7 and the incoming call quality is awful. It sounds hollow and muffled and hard to explain but like I have to chase the sound around the phone with my ear. There's a temporary sweet spot whereby if you move the phone around you can find a better quality moment but it doesn't last.

Altering the volume down does help the quality for a bit but it doesn't last either, the call seem to kick back into a hollow tone and I have to up the volume. The constant up and down is very frustrating even on short calls.

My case hasn't arrived from Apple yet and no screen protector so there's nothing blocking the speaker vent. Hasn't been dropped or near water.

Any ideas?

I've tried changing volumes, turning off and on again (Apple support suggestion), changing the Hearing Aid accessibility to on in hope of terminating the inbuilt noise cancelling and moving the phone around. Nothing has helped.