Call Failed and WiFi Button Greyed Out

Anytime I pick up a call or someone picks up my call, I get a "call failed" message. It's been happening for the last couple of days. I tried turning off/on 1) my phone, 2) cellular connection, 3) airplane mode, and 4) WiFi. I installed the latest iOS and reset all my network settings. Now, even my WiFi button is greyed out, so I cannot even use that. I tried calling AT&T, and I did everything they told me to try except buy a new phone. Is there anything else I can do? Thank you!