UK mobile operator O2 is teasing the British public with warnings of limited availability of the iPhone 3G, when it launches on Friday.

On Tuesday the carrier confirmed that excessive traffic to its web site on Monday caused the order system to fall over after it started taking pre-orders for the device.

The entire stock of pre-order units was depleted within a few hours, O2 said, without revealing how many devices it sold.

On Friday, July 11, O2 and Carphone Warehouse will open their retail outlets to the general public at 8.02am, expecting demand of “unprecedented levels”.

The Apple device will be sold on a first come, first served basis to both new and upgrading customers in all retail outlets. “We’ve never seen any mobile device create the excitement and demand of the iPhone 3G,” gushed Ronan Dunne, CEO of O2 UK. “We want to ensure that everyone who wants an iPhone 3G can get one so we’ll be working with Apple to continually replenish our supplies throughout the summer.”

O2 also officially confirmed that prepay iPhone users will be able to get unlimited mobile data and wifi access for £10 per month, as reported by telecoms.com already. However, the company also said it would not be selling the device itself as a standalone prepay product until Christmas.

Interestingly, in order to push adoption of the device, Apple and its operator partners are actively encouraging upgrading iPhone users to give their old 2G device to friends or colleagues and keep them in circulation.