The best prepaid phone card I have found is one sold by Costco (I think it is issued by MCI} for $18-20 for 200 minutes. It has worked at all USA pay phones so far, and most importantly, works at USA hotels often without the $1.00-2.00/call charge tacked on by (mostly) the quite greedy business and luxury hotels. I also use it when my ISP doesnt have a local assess number. Works great.

You must belong to Costco to purchse or get a friend to buy one for you. I will try to see if they are valid outside the USA (I have a trip to Vancouver next week and a trip to London the week after that).