I've got an at&t s8+, and starting about 3 weeks ago I couldn't call any iPhone on ANY carrier. When I would hit dial on my end, the screen would say calling for about a second (long enough for it to start ringing on their end) and immediately drop the call on my phone. Half of the time of they tried immediately calling me back, it would go to my voice mail without ringing on my end. Now if they originated the attempt on the iPhone end, I could receive the calls just fine. Texts worked just fine both directions. After trouble shooting everything I could think of (clearing cache from recovery page, clearing cache in all 3 phone dialer system apps, booting into safe mode, factory reset) and speaking with 2 different reps from 2 different stores (who were beyond useless, and had ZERO knowledge of anything if you don't have the holy grail iPhone..... excuse me while I throw up....) anyway...I figured out at least a temporary fix until an update comes through:
Go to settings, connections, mobile connections, enhanced LTE services, and uncheck the advanced HD voice and advanced communications box.... instantly calls to iphones worked perfectly..... hope it works for you guys.