Easy, and I am 99.99999999999999999% sure you have not being charged for a PXT as they are free. If you were being charged every other prepay customer would be and that would be a serious issue. there were lots and lots of tests done before it went free so a issue like this would not happen
On my computer at work. I do not have to go into the details but it is very easily done. Something else you could have done to be charged is play around with the POP3/ iMAP4 Email client in the handset, because if you send/receive on that, it will have to start a GPRS session. Even thought it may not be set up correctly it still logs onto the GPRS network and starts a session. There is a interval time that can be set up on these that people do not realise they turn on, and this can be set like 5 min 10 min 1 hr and so on.