How to update iPhone os version

An iPhone 6 cannot be upgraded beyond 12.5.6.

But that's not what it's telling you, it's telling you that you need to update your carrier settings, not your operating system.

Update your carrier settings on your iPhone or iPad

Manually update your carrier settings on your iPhone or iPad - Apple Support

You are looking at information about an app that requires a higher version of iOS than your iPhone is capable of handling.

The iPhone 6 can be updated only as far as iOS 12.5.6

If you need to run a higher iOS version, you will need a newer phone.

Contact your carrier regarding the problem with not being able to make a call. This is a separate issue. Only your carrier can help with issues regarding making or receiving calls.