3G Using Up Lots Of Time Credit

[BIGJIMMY]

I have an iphone 4s and am on 1-2-call pay as you go, regarding the 3G at the moment I still have around 20 hours left that I brought on a package, as I understand it when a wi-fi signal isn't available the phone kicks into 3G (if aviailable) and this time is deducted from my 3G credit, all of this is assuming that the wi-fi is turned on and also the Enable 3G and Cellular Data in settings is on, my query is this, if I'm on 3G for a few minutes the auctual 3G credit that I have goes down by alot more than a few minutes, I mean I use it for 2 minutes and the credit had decreased by 7 or 8 minutes, I have spoken to the operators at

1-2-call who have said that either my phone is faulty or i'm using more minutes than I thought, the credit isn't connected with date transfer only time, because of this I keep the Enable 3G and Cellular data off and only tuen them on to check e-mails etc then turn them off again - sort of contradicting the whole point of having a 3G service on the phone.

Am I doing something wrong, is 1-2-call wrong or is my phone faulty?

Thanks in advance.

Jimmy