Will I get charged for using data once on an unlocked iPhone?

So here's the deal:
I have an unlocked iPhone 3GS on T-Mobile, but I do not have a data plan. I use Wi-Fi instead.
The other day I momentarily turned on Cellular Data in Settings in order to send a picture message, because apparently that has to use data, but I turned it off right away after it had sent.
Will T-Mobile now know that I have an unlocked smartphone and start charging me for a data plan? Before this, it just continued to charge me as if I still had my horrible 4-year-old Samsung brick.

I will probably have to cancel the service if they do start charging. Also, if they do, is there anything I can do about it?

Any help is appreciated!

IPhone Factory Unlock means you will be able to use your iPhone to different carriers without any issues. Example, you are currently on TMobile and if one day you plan on changing to ATT, you can just insert an ATT sim card and you can automatically use your phone to ATT network.

Using the Data is different from Unlock.

If you are connected to a Wi-Fi while browsing the internet then your carrier won't charge you any data on it. Just remember to turn off your Cellular Data. But if you turn it on, and you are not connected to any Wi-Fi network and you send a picture or browse the internet then your provider will surely charge you for that.

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