Apple has introduced new APIs in iOS 26 allowing developers to implement support for making cellular phone calls and sending/receiving SMS/MMS/RCS messages in third-party apps, but only for iPhone users in the EU.
Previously, such capabilities were limited to Apple's Phone and Messages apps, making this a significant change, possibly in response to the EU's Digital Markets Act.
To utilize these features, iPhone users in the EU must select a third-party app as their default dialing app and/or default carrier messaging app.
These capabilities will not extend to third-party iPad apps or iPhone apps on macOS or visionOS, and for now, the functionality is exclusive to the EU and requires an Apple Developer account registered in the EU.