U.K. wireless carrier O2 took preorders for Apple's iPhone 3G on its Web site Monday morning, and within the first hour, the volume of traffic caused the site for upgrades to crash.

The mobile operator sent potential customers e-mails and text messages saying that they could secure a device at 8 a.m. local time. The company said successful preorders would be delivered on launch day, July 11, and could be activated at home with iTunes.

But the flood of customers caused the company's automated upgrade site for existing iPhone owners to slow to a crawl, and eventually crash. The increased volume of traffic didn't take down the company's preorder system for new iPhone customers, though.

Despite the technical glitch, the wireless company sold its entire stock of preorder iPhones. "Due to the huge demand for iPhone 3G, we're currently out of stock online," read the company's Web site.

The iPhone 3G will still be available in stores this Friday, and the company said that half the handsets it ordered were made available for preorder customers. The wireless carrier didn't say how many smartphones it expected to stock in total.

The increased demand is not completely unexpected as 02 said more than 200,000 people registered an interest in the iPhone 3G. By comparison, the original version had 35,000 people register interest before it went on sale.

The wireless company previously announced that customers could get the 8-GB version of the highly anticipated device at no additional cost with a service plan priced at either $88 or $146.50. The 16-GB iPhone 3G would also be available at no additional cost with the more expensive plans.