What you have is an iPhone 6 with No service but is connected to a WiFi Network. You also have an iPhone 12 with an active service and it is connected to the same WiFi Network. Both iPhones are also signed into the same Apple ID. You also have Calls on Other Devices Active on your iPhone 12. This is why your iPhone 6 is ringing along with your iPhone 12 when both iPhones are connected to same WiFi Network and are nearby. This is part of the handoff feature of iOS.
If all of my assumptions are correct above then you can stop this behavior from your iPhone 6 by doing the following:
On your iPhone 12. Open Settings ➔ Cellular ➔ Calls on other devices: Turn this feature OFF.
OR
On your iPhone 6. You can sign out from your Apple ID and iCloud and then it will stop ringing.
Axel F.