I generally explain to the CSR I am on the phone with when I need to make an unpleasant phone call:

    Look, I know this isn't your fault. I know you didn't cause this, but I need to be angry at someone. 
    If you could redirect my call to the right person it would be greatly appreciated.

I generally either receive very good service (a thank you goes a long way), or I get forwarded to someone whose job it is to deal with "problem" customers and we have at it.