I had an experience leading me to a simialar idea. I bought two 60 minute cards and tried putting one on my phone. Well all I got was invalid code. I tried the other one. Same, I took it back and exchanged them for two different cards after harrassing the people at Walmart for an hour about it.I got home, tried both cards again, and no go. I called the people at TracFone the next day. I told the guy my story and without talking to anyone else or even giving him my ESN, he gave me four different codes and it put about 300 minutes on my phone. This leads me to the conclusion that ESN has nothing to do whatsoever with the codes you put in for your minutes. Also, there has to be some universal codes just to stick in the phone to give it minutes. From now on, I just call the people there, tell them about the same story, and get minutes.