I have exactly the same issue. I had an issue of calls dropping after 8-9 seconds on an iPhone 13 Pro. I handed it in now via Apple Care to get some diagnostics. In absence of my iPhone 13 I switched back to an iPhone 11 where none of these issues occur. My wife recently bought an iPhone 13 mini to replace an iPhone 8, and it has exactly the same issue. She handed the newly bought iPhone 13 in and bought an iPhone 14. Guess what: same issue. She switched back to the iPhone 8, where no issues occur. We contacted our provider: they don't see any issues on the network, which is logical, because the symptoms don't occur on an iPhone 8 nor on an iPhone 11. So to conclude so far: we have 3 iPhones (13 Mini, 13 Pro and 14) with dropping calls after 8-9 seconds. We switched to eSIM, didn't solve it, switched back to a new physical SIM, didn't solve it neither. iPhone 8 and iPhone 11 work perfectly well. All iPhones are on the same version 16.3. I'm really surprised to see "only" a few hundreds of people having this issue. We had a thought that maybe the dropping calls are linked to the fact that all our devices are connected to apple watches and iPads... The reason why I mention this is because we have an impression that the issues are more frequent at our home location, and less when we are away from home... This is really a weird issue. On top of all of this: we have two family members with an iPhone 13 who don't have the issue... Symptoms are the same with WiFi active or off. We are on a 4G network. Calling via WhatsApp works fine on all devices. APPLE, this is a horrible issue that requires a quick fix!