iPhone 11 call issues after iOS 26 update: Persistent incoming and outgoing call failures

Following the iOS 26 update, the iPhone 11 has experienced persistent calling issues. In contrast, my iPhone 15, which has been running iOS 26 since its release, has functioned flawlessly. However, both of my parents, who own iPhone 11s, have experienced similar problems with their phones. They have noticed that their phones randomly lose the ability to receive incoming calls, and when they attempt to make outgoing calls, the calls do not ring. This issue affects both incoming calls from the Android app and calls made directly from the iPhone. The problem is entirely unproductive.

Both phones are up to date running iOS 26.1 (23B85)

I've done a few things to trouble-shoot:

Force restart iPhone - Apple Support > Issue temporary fixed; issue came back after a few days.

Reset your network settings > Issue not fixed; nothing changed.

If your call fails or you can’t receive calls on your iPhone - Apple Support > Issue not fixed; nothing changed.

[Re-Titled by Moderator]

Original Title: iPhone 11 Call Issues