AT&T warns users they’ll be charged if the continue unauthorized tethering

AT&T confirmed Friday that it has contacted a small number of its smartphone users and ordered them to stop tethering their phones without paying for AT&T's tethering service or the carrier will automatically add tethering to their monthly bills.

"We are sending emails, letter and text messages to a small number of smartphone customers who use their devices for tethering but aren't on the required tethering plan. Our goal is fairness for all of our customers," an AT&T spokesman said in an email.

The alerts give users the option to stop tethering and continue on their current plan, call AT&T and add tethering to their plan, or risk having AT&T automatically bump them to a tethering plan.

AT&T said the warnings impact a "small percentage of our smartphone customer base."

According to Ars Technica, users have until March 27 to make their choices, after which the automatic tethering plans will go into effect.

AT&T smartphone users have the choice of 200MB for $15 per month or 2GB for $25 per month. A DataPro plan adds 2GB for $20 extra per month for a total of 4GB for $45 per month.

Earlier this year, AT&T launched its first smartphone with a built-in mobile hotspot, the HTC Inspire 4G phone. Apple's iOS 4.3 update, meanwhile, added a hotspot feature to the AT&T iPhone 4.

However, smartphone users - particularly those on the iPhone - have long looked to jailbreaking to allow for tethering. After the 2009 launch of the iPhone 3GS, for example, Austrian blogger Benjamin Miller posted a tethering workaround for the iPhone that allowed you use the iPhone to connect to the Web either via USB or Bluetooth. At the time, AT&T said it could "not guarantee the performance of any app we don't offer."

AT&T did not say how it was identifying the tether offenders, but Ars speculated that AT&T was just looking at peoples' data usage and making an educated guess. They also pointed to TUAW, which said browser ID strings might be a giveaway.

Sara Yin contributed to this report.

This article, written by Chloe Albanesius, originally appeared on PCMag.com and is republished on Gear Live with the permission of Ziff Davis, Inc.