I'm making an app and i'm curious: is there any way to know how much money does the user have on his SIM without the need to dial it on the phone app? For example, in Portugal i have to type *#100# to know how much money i have and how many messages i have left.
I think you can not do it. The SIM access requires the use of AT commands, and this is not included in the official SDK. However, you may develop the app for jailbroken devices. Also I'm not sure that the credit is stored in the SIM.