On Call: How does my carrier know which phone I'm using?

Q: Recently I bought a used iPhone 3G from a friend. A month after I started using it, I realized that AT&T added a data plan to my service. I never use the data features so to get around the requirement, I accessed my customer profile on AT&T's Web site and changed my phone to an old Sony Ericsson W760i. I then called AT&T and asked them to take the data plan off and they reported seeing the W760i in my profile.

About an hour later, though, my profile said I have an iPhone again! Now I suspect that in another month they will tack on the data plan again. Is there a certain jailbreak that will stop AT&T from seeing that I have an iPhone?
- Ryan

A: Unfortunately, each time you use your iPhone 3G, AT&T can tell which handset you're using. The company gets this information not from your customer profile, but from your phone itself. Here's how it works.

Each time you make a call, a couple of things are going on. First off, your SIM card identifies you as a subscriber to the carrier. If your account is in order, then you can make the call. Before that can happen, however, the network also accesses your phone's International Mobile Equipment Identity (IMEI), which is a number that's unique to every GSM handset (CDMA devices use something similar called an Electronic Serial number, or ESN). The IMEI identifies the phone to your carrier and it checks that the handset is valid to use. If the phone is locked to another carrier or if it's lost or stolen--your carrier can use the IMEI to "blacklist" a device--you won't be able to make calls.

So it's the IMEI, and not your customer profile, that lets AT&T know that you're using an iPhone. You can switch to the W760i for a while, but once you go back to the iPhone AT&T will know. I'm not sure how how much time the carrier waits before tacking on the mandatory data plan, but it will happen eventually. I'm not in favor of mandatory data plans, and I really don't like it when they add new services to your contract without asking, but it happens all the time. Unfortunately, jailbreaking won't help since that process only removes the Apple-imposed restrictions that block you from downloading third-party apps.

You can find your phone's IMEI by looking behind the battery cover, on the box, or by typing *#06# on your keypad.

Q: With Windows Phone 7 making all Windows Mobile 6.5 devices effectively obsolete, I'll be stuck in the middle of a contract and unable to upgrade my phone without it being very expensive. Obviously, this could slow sales of Windows Phone 7. Do you think Microsoft might try to do something to defray the cost of going to new hardware as a way to help sales (e.g.: convince the carriers to allow earlier upgrades)?
- depinka

A: Microsoft could work with the carriers to offer deals to consumers, but the carriers really drive those decisions since they're the party that buys the phones in bulk and resells them. Yet, carriers have an incentive to offer customers earlier upgrade terms. Not only would users be locked into another two-year contract--not surprisingly, carriers really like that--but also the carrier would be able to move more handsets. If it does happen, I just hope carriers refrain form slapping on one of those painful upgrade fees. On the upside, once you do get a Windows Phone 7 device, OS updates will be sent to all devices at the same time. That's a big advantage that Windows Phone 7 has over Android.

On a related note, U.S. Cellular recently instituted a new Belief Program that awards longtime customers with quicker updates--as short as 12 months in some cases. I'm glad to see U.S. Cellular reward, rather than punish, loyalty so I hope other carriers will follow.

Q: I am 78 years old and wear hearing aids. When talking cell phone to cell phone I cannot hear very well on my Motorola. My contract is up so I am getting a new phone. What would you recommend?
- Leola

Most cell phones are compatible with M4 hearing aids and a few are compatible with T4 models, as well. Usually, you can find this information by accessing the specifications page for a particular handset on a carrier's Web site. Verizon Wireless, for example, is very good about making hearing aid compatibility readily available. I'd also suggest visiting your local carrier store and inquiring about the exact compatibility for the models you're considering.

Q: I live in a city with 4G WiMax from Clearwire and Sprint. I have a 4G USB modem from Clearwire for my laptop and a 3G/4G mobile Internet plan. Also, I recently bought Sprint's HTC Evo 4G. My question is, should I dump my Clearwire mobile Internet plan in favor of the Evo's hot spot function? I think they are using the same network so will I see any difference in performance? The Clearwire service costs $55 per month and the hot spot fee from Sprint is only $30 a month on top of my calling plan.
- Mike

A: I see why you're considering this, particularly since it will save you money each month. Indeed, you could make this work, but there are a few points you should keep in mind. First off, you can get simultaneous voice and data only when using the 4G network. If you're using 3G, you won't be able to make calls and use the data features at the same time. Also, your reception during peak hours may not be reliable. When Bonnie Cha reviewed the Evo,, for example, she found that the Internet connection between the Evo and her laptop dropped a couple of times during periods when a lot of other users were on the network. Also, she had trouble streaming videos.

In Bonnie's experience, the Evo averaged download speeds of 3.42Mbps and upload speeds of 0.93Mpbs and reached a peak speed of 4.76Mpbs. On the upside, setup was easy and there's no data cap, but the process involves a step beyond just plugging a USB model into your laptop. If you're comfortable with those caveats, then you could go for it. But if you're going to be a heavy laptop user who depends on a reliable data connection throughout the day, I'd be a little wary. True, there are a selection of third-party apps that will offer free tethering to your Evo, but you'll still be dependent on Sprint's network. Perhaps you could test it first to see how it performs. On that note, I'd love to hear suggestions from any CNET readers who've used the Evo's hot spot feature regularly.