I bought a Challenger phone card at £5. Normally I should get at least 140 minutes calling Zambia from England. I made the first call and I talked for 17 minutes and when I tried to make a second call the credit was quoted insufficient! I called customer services twice and each time I was told to call in 6 hours because apparently my calls were not updated yet.
It was like talking to a machine that repeats please call back, please call back.
I would advice not to buy them.