A good amount of excellent advice is given in: http://www.flyertalk.com/forum/Forum1/HTML/001535.html . In a nutshell, dialing 10-10-636-1 and then your interstate number gives you 5 cents/min 7/24 in most states. No fixed monthly charges either. If you don't wish to do that, many other plans are listed for your consideration if you wish to do a little research.

Billings have been accurate and my savings in a couple of months are over $400. Good Luck!!!