HIGH International Phone call charge! $5 per minute!!

I have FIOS triple play and Vonage IP phone which I am using for my international calls. One late night during June 2010, I received an emergency call and dialed back (incidentally) used the wrong phone. I should use Vonage phone but instead had used Verizon phone for about 20 minutes.

I didn't realize this mistake until I received this month bill charging me extra 100 bucks for this 20 minutes call at about 5 dollars per minute.  I think it is very high charge for me to swallow; I have tried to contact customer service without any help so far. What should I do next? I feel helpless at this point. Thanks.