On my iPhone 8 running iOS 12.1, under:
- "Settings / Cellular / Calls on Other Devices",
- "Settings / Phone / Calls on Other Devices",
- "Settings / Messages / Text Message Forwarding"…
…I see two list items for test macOS user accounts I created and deleted in troubleshooting another issue.
These two non-existent devices don't appear in the list of devices:
- at the bottom of "Settings / Apple ID",
- on https://appleid.apple.com/account/manage,
- on my Mac under "System Preferences / iCloud / Account Details / Devices".
I've tried toggling the faux devices, toggling "Calls on Other Devices", and rebooting the phone and the notebook.
I assume now, in retrospect, that I should've logged out of iCloud on the test macOS user accounts before deleting them (including their home folders), but I'm pretty sure now that's no longer an option.