If I am a trustee of my father's estate, am I entitled to disclosure of his accounts from banks and stock companies?

Is your father still living?  I ask because a Trustee is usually the person who administers a trust.  The assets in the trust can be called the "trust estate," so it is possilbe that your father is alive and you are the person in charge of his trust.  If you are this person, you are entitled to disclosure of everything in the trust, but not necessarily everything he owns.

On the other hand, an "estate" is often the assets of a person who has passed away.  In that case, the person in charge of those assets is called a personal representative, executor, or administrator of the estate.  If you are this person, then you are entitled to disclosure of everything he owned.

It sounds like you are trying to get banks and stock companies to send you information.  Try sending them the document that appointed you as "trustee" (or representative).  If this is a trust, you can send a Trust Certificate or the full Trust Agreement.  If this is a probate estate, you can send your Letters of Administration.  If that does not work, you will have to consult a lawyer to sort this out.

Good luck.