Galaxy S6 :: Answering A Call Seems To Hangup / Drop The Call

I've had my shiny Galaxy S6 for nearly three weeks now. I bought it SIM free. Really loving the phone, but I've noticed that when I try to answer a call, the phone hangs-up instead. I have good signal and I can ring the person back immediately. At first I thought I was hanging up with my cheek, but it happened when I was in slow traffic in my car yesterday. The phone was sat in a mount, connected to Bluetooth. I used the car's controls to answer and had the same issue.

I've spoken to people who call me, and they say the call just ends with a beep, it doesn't go to voice mail. I've tested calling my S6 from another mobile and a land line and the problem happens every time.

If I tap reject, the call is sent to voice mail, so I'm not pressing the wrong button.

I've tried:

-Enabling/disabling single tap mode
-Enabling home key to answer calls
-Wiping the phone's cache partition using the Android system recovery menu
-Using a Bluetooth headset to answer calls
-Uninstalling Three InTouch, my network's Wi-Fi call app (I can answer calls if they come through this app)

I'm in the UK. Phone is SM-G920F ZKABTU, baseband G920FXXU1AOE3, CSC version G920FBTU1AODQ. I haven't rooted it.