I visited my mom in the SF Bay Area in July. I have an Android that I use habitually, so before leaving I checked if she lived in a covered area (T-Mobile’s data coverage map has her in a “Very Strong” area surrounded by “Excellent” with no gaps) and, just to be sure, I went into my phone settings and unchecked “Data Roaming: Connect to data services when roaming.” (Mostly, I was worried about automatic data use during the drive down.) For the record, I have their $10/mo Unlimited* 2GB plan, which comes with 50MB of roaming data. T-Mobile also has a 5GB plan with unlimited roaming for $25/mo.
On 07/27, I received a text message: “You’ve used up 40MB of 50MB allotted for data roaming. You will be limited to 50MB.” I called T-Mobile, confused, and the representative told me that “I don’t see this on your account” and “this is probably just a system error.” So for the next day, I kept using my data plan. (And I stayed at the apartment because of the Olympics, so I couldn’t have left the service area.)
On 07/28 I got a text informing me that I had reached my limit. The web stopped working. I called customer service again and the rep told me that in order to keep using data, I would need to upgrade to the 5GB plan. I told her I didn’t want it and that, had I known I would be roaming, I would have been more careful. Besides, I was going home again in three days.
I asked why I was roaming in the first place if the setting on my phone was off. Here’s the kicker: the Data Roaming option only controls whether or not you roam while traveling internationally. What T-Mobile calls Data Roaming is not what my phone calls Data Roaming. I asked if there was an option to turn off accidental data roaming while in-country. She suggested I turn off my phone. But aside from a supposedly erroneous text message, I had no reason to believe I was roaming, so why would I have done that?
Check your phone’s manual (or user forums) to find out whether there’s a way to disable mobile data when you don’t need it, especially if the phone has wifi access.