[GadgetKen]

No callback sim card is perfect. Have tried travelfone(out of business), UM, UM+, and Yackie. Pricing isn't always what they say on their website, callbacks sometimes fail, and the audio quality isn't always perfect. I'd say probably say 9 out of 10 calls work perfectly and there's something amiss with the tenth call.
That said, it's been much cheaper for me to make calls back to the USA from other countries using a callback sim, country or regional specific sim, or a satphone than using a hotel room phone (typically US$3.49/minute from non-US Caribbean on C&W CallUSA service which is still cheaper than AT&T Direct) or a postpaid mobile phone(US$1-US$3 depending on carrier, country and rate plan).