https://wccftech.com/apple-releases-ios-15-1-1-with-call-improvement-for-iphone-12-and-iphone-13-series-only/

Today, Apple has seen fit to release iOS 15.1.1 that fixes call-dropping issues on the iPhone 12 and iPhone 13 series. Take note that the new update is only available for the aforementioned iPhone models only and all other models will not see the update in the Settings app. The new update arrives almost a month after the launch of iOS 15.1 and can be downloaded on supported iPhone models right now.

As mentioned earlier, Apple has released iOS 15.1.1 for iPhone 12 and iPhone 13, including the Pro models. If you happen to own the supported models, the update is available for free from the Settings app over the air. All you have to do is launch Settings > General > Software Update and you are all good to go.