Will iMessage use my prepaid credit?

I recently bought an iPhone 4s on prepaid and it currently has an Optus sim on the 2 dollar a day plan/cap whatever you want to call it and I was just wondering if it will use my credit balance? I have unlimited text, calls and mobile data. My girlfriend recently experienced a problem with this after she activated it and it chewed up all of her credit balance, she is on the same prepaid service as me.