I think that the simplest thing to do is just buy the local Taiwanese telecom cards (the ones with the interlocking handset logo on the top of the card). The ones that I get cost NT$100 each, and are specific for the ubiquitous card-phones that take them. For calls back to the US, they seem to last about 4 minutes, which is comparable to the rate you found. There is a series of 11 dots along the top of the card creating 10 10-NT$ divisions, which is the billing unit. If your card expires during the call, it is ejected at the beginning of the last billing unit, and you have about 25 seconds to withdraw the old card, and insert a new card, to continue the call uninterrupted.