Hi Donald1946,
Thanks again for sticking with me on this one.
As the other handsets are ringing out and not getting the same error, this would point to an issue with the handset that is experiencing the error as opposed to the phone line itself.
I would recommend resetting the affected handset, maybe try unplugging and seeing if this resolves things. If it doesn't I would have a look at the manufacturer handbook for the device or alternatively buying a new handset.
Let us know if you have any further issues.
Thanks,