Conversation so far:
When was Mielke imprisoned?
1989.
What crime was he convicted of?
ruling that Mielke was not mentally fit to stand trial.
How long was Mielke imprisoned?
1,904 days.
What happened during his imprisonment?
Mielke enjoyed having imaginary conversations with non-existent Stasi agents.
Are there any other interesting aspects about this article?
[Mielke's] bank account, which held more than 300,000 marks (about US $187,500), was confiscated.
What did Mielke do after being released from prison?
all further prosecution of Mielke had been indefinitely suspended.
What else is interesting about his imprisonment?
During his incarceration, at JVA Moabit corrections officers supplied Mielke with a red telephone like the one in his office at Stasi Headquarters.

Question: What did he use the telephone for?
Self-contained rewrite: What did Erich Mielke use the telephone provided by Mielke during his imprisonment for?