iphone 3gs, skype, skype credit after 2 calls

[ridnlow]

Hi all, I downloaded skype from the itunes store. I tested it last night, worked, was happy to use it while on wi fi to reduce my phone bill minuets used. I called this morning worked great, now this afternoon i tried to call twice while on wi fi signed in, and now it comes up and says I have to purchase skype credit to make the call ?? I tried the same number I called earlier and different numbers ? so skype is not free as most say ?

After doing more research I found the "I call VOIP" app and it has worked, anyone know if there will be any surprises on my phone bill from using i call???