In court cases, telephone conversations are sometimes used as
evidence.  Are all conversations taped by the phone companies or is it
only possible to obtain this evidence when the phones are tapped for a
specific reason?

i work for a phone company and we definately do not record any
conversations.  Tapping is the only way.

As a rule, only a court-ordered phone recording would be admissable as evidence.

Are you sure, Myo? I did a lot of legal reseach in the 80's and
clearly remember a case in the NW 2nd Reporter where a man put a
recorder on his own phone, and obtained a tape of his wife conspiring
with her paramour to do him in.  He attempted to use that recording to
obtain justice in the divorce courts.  The District Court judge said
it was not admissable, because state law prohibited recording
telephone conversations when neither party was aware of the recording.

On appeal the high court said that made no sense since the crime
involved was so serious.

My point here is you need to talk to an attorney in the relevant
jurisdiction. See google legal warning below.  Each state may be
different, and it als differs what type of case it is.

Clearly for a criminal conviction, the rules will probably be tighter
than ofr other types of cases.  Better get appropriate legal advice.

Irlandes, 
Obviously I am just a lower court judge.  ;-)
Not a valid source for legal advice.