I have been collecting phone cards since 1992. During that time, I have collected several hundred phone cards, mainly mint cards produced by Telecom New Zealand.
In the early nineties, the value of phone cards rose dramatically as Telecom produced limited numbers of its cards. This caused more people to take up the hobby, which caused the values to rise, which caused more people to take up the hobby, which... well, you get the idea. The bubble burst in 1994, and now days it is possible to get phone cards cheaply on Trade Me. Phone cards which would have cost several hundreds of dollars in the early nineties can now be bought for thirty or forty dollars.
What are my favourite phone cards? Telecom's Disney cards, particularly the Stars of the Silver Screen cards.