[Windows Phone 7 General] N8ter:
I was just on AT&T. This is all I will say, because I've been through this.

My Mom had a RAZR. She put her SIM Card in a BlackJack, and AT&T's system automatically added the data plan to her bill.

Your phone doesn't need to use data, they will know by the IMEI and other phone information that you have a smartphone. Everytime you phone connects to their network, it sends out the IMEI and the system will check that IMEI against you account. If you have a smartphone IMEI on a dumbphone plan, they will automatically add the smartphone packages to your account. You'll even get a text saying that you put you SIM in a smartphone yada, yada. The only way to have a smartphone without a data plan is to buy it outright off contract and activate it only with a voice plan on it.

There is no way to get around that. Swapping SIMs to make calls won't avoid that. Only modifying you phone IMEI and making it look like a dumbphone will do that. The process is automatic and happens instantly. Put you sim in, turn you phone on, turn it off... Too late. You have a smartphone plan on your account now.

My mom never used any data on her phone (no ActiveSync account setup, never used the browser... we checked for data usage - there was none), but AT&T simply told her if she puts her contract SIM in a smartphone, they have to add the data plan. She would have had to ETF her contract if she didn't want to pay for a data plan, so she went back to her dumbphone.

And to expand on some things, just in case you don't understand where I'm coming from:

1. That's impossible. AT&T (any cell carrier's) network checks the IMEI/ESN of every phone that connects to their network the minute it connects. They will know exactly what type of phone you have. It has absolutely nothing to do with what operating system is on the phone. It can be a WP7, WinMo, Droid, iOS, iBangedUrMom device... They will still know exactly what type of phone you're using the minute it connects to their network via the phone's IMEI (and probably even the Serial Number, but they shouldn't even need that).

2. See #1. The phone doesn't have to access data for them to know that you're usin a smartphone on a dumbphone plan. All it has to do is connect to their network, and they'll know by the IMEI that you're trying to gib them. They will then alter your plans as required to suite their smartphone requirements (Add Data Plan, etc.).

BTW, when AT&T automatically adds data plans, they add the highest data plan. So, even though you don't use data at all, they will add a $25 2GB data plan, not a $15 200MB data plan.

And there's nothing you can do about it. It's in the contract and the requirements are pretty much universal across carriers here.

No carrier allows you to have a smartphone on a dumbphone plan (a plan without a data package) if you're under contract. The only way to do it is to buy the phone outright and have a plan without a contract. In that case, you'll just have to pay overages everytime you use data - which can easily (and quickly) exceed the costs of a data package.

And TBH, it's not even worth it to use a smartphone without a data package, even if you have WiFi in most cases. I use my HD2 as an iPod of sorts right now and there's no way it would be usable without a data package even though I have WiFi in most places I go. Missing an email I could have checked at a red light telling me soandso isn't feeling well and isn't going to be there, so I can spare myself the 45 minute drive is just not acceptable for me.

That bull**** about them just removing the data plan is false. That's what warranty is for and if you're a dumbphone user you should be getting a brand new dumbphone everytime you contract is up for renewal basically for free. Either you get a new one (they can give you a loaner phone until then, and it won't hurt since you use dumbphones, right?) or pay the price. They won't let you breach you contract. That would be negligent of them, and unfair to all of their other users.

If they do take it off (probably assuming you will take the SIM out) the system will automatically add it the minute you turn the phone back on - and the next CSR may not be as willing to break the rules (and none of them can just "change" the system).