I am not sure, they give the sim card and together with the sales person we went to some kind of ATM machine she typed in my phone number plus pass code and I put 300 rmb in it and got 300 rmb credit on the card, so in reality I did not pay anything for the sim, would that be possible? Is there a number to deal so I receive an SMS with the credit balance on the card? I did not take the wifi one, because I can use my own one from home to log in, by local cafés etc

Try this China Unicom store. Pretty large and well staffed. You will need passport.

goo.gl/maps/Ejux4

China mobile: Call 10086111, it's an automated line, so as soon as they start talking, hang up. You'll receive a text back telling you how much you've used this month and how much is left. If you have internet you'll get a separate text with data usage info. Of course it's in Chinese, but it's just the numbers you'll be looking for :)
I need internet on my phone for when i'm out and about, sitting on the bus, bored at work etc etc, i can't live without it. Haha.

Thanks, and it looks like I had the sim for free, I had made two calls and there was still 295 rmb on it

@mPRin
Very useful, thanks. I knew the number used to be 10086, but that hasn't worked for a while. Cheers.

10086 still works for text balance info. Text 'ye' to this number (obviously without the quotes) and you will get a text back in a jiffy with your balance and your usage for the week, although I just now used this method and only received my balance.

If there are two numbers though, the first is your balance.

Oh, that's cool. I tried it with the "ye" text and it took only 2 seconds to get a reply.

@AlPage48 : I lied then - a jiffy is supposed to be 1/100 of a second! I beg forgiveness!!