I recently inherited this phone, and have the same problem.
But I found one phone socket in my house that did not have this problem,
so it seems to be a combination of a fault in the phone and some characteristic of the phone line.
Then I tried putting an old DSL filter (NetSpeed EZ-DSL inline microfilter)
between this phone and a phone socket which didn't work right in earlier tests.
That fixed the "failure to disconnect" problem!
I don't have DSL service, so it's a mystery why this works.
I hope that this fix works for you.