Apple today released iOS 15.1.1, a minor update that comes almost a month after the launch of iOS 15.1.
The iOS 15.1.1 update can be downloaded for free and the software is available on all eligible devices over-the-air in the Settings app. To access the new software, go to Settings > General > Software Update.
According to Apple's release notes, iOS 15.1.1 improves call drop performance on iPhone 12 and 13 models, so users who have been experiencing dropped calls should see improvements after installing the update. It appears to only be available for devices in the iPhone 12 and iPhone 13 families, including Pro models, so users with other devices won't see the update.
Source: Macrumors