New iPhone 11 acting up during calls

It's QUITE annoying and I've never had this problem with an iPhone before. If I take calls and have my phone up against my face, my phone will often mute itself, start dialing random numbers, or even initiate a facetime session. What is causing this, and how can I fix it? I've read it might be a problem with the proximity sensor, but my phone is brand new--purchased mid-March. Would the case it's in have an effect on the sensor? Help??!