Can you insert a sim from a dumbphone into the new iphone if I have T-mobile?

I'm trying to avoid a data plan. I know AT&T detects if you are using a smart phone, but I heard T-mobile doesn't. And I could just use Wi-Fi. Is this possible, and could I cut the card to fit? Is there any way at all to do this?

Whoever told you T-Mobile doesn't, is wrong.

There's absolutely 0 way to avoid a data plan.

I do the same thing with my Samsung Galaxy Nexus on T-Mobile. I pay by buying money cards for T-mobile and have data disabled. I pay a flat rate for Texts/SMS/MMS. You would have to cut the sim card down to Nano-sim if you are using the iPhone 5/5C/5S.

The iPhone 5/5S uses a micro sim card, which is most likely not the same type of sim card used in dumbphones. You can't cut it to fit, and unfortunately you will have to pay for a data plan. You could try switching to another contract free carrier such as Virgin Mobile (their smartphone plans start at $35 a month for unlimited texts and data and 300 minutes).

You could do that on any carrier with any phone, but this will not help you avoid a data plan.

The SIM card I used in my iPhone 2G & 4 was the same I used in a Nokia 5100 & 6230i before hand.