My personal experience has been that (well-written) letters get a better response than any phone calls - at the least it gives you a chance to properly formulate your thoughts into the best presentation whereas on the phone you never quite know what youŽll come up against.
When a company goofs, particularly if it is one I use frequently, I write them a letter in this 3-part format:
1. I point out how much IŽve been using them and explain why.
2. Narrate (clearly and briefly) what happened.
3. Express my outrage at this behavior and point out why it is unacceptable. I then conclude by saying what I think they need to do to rectify the problem.

If a letter is written well and particularly if it comes from a regular customer, the company will usually do its best to make you happy.
Yonatan