Apple’s (AAPL) British-accented partner reported yesterday that the iPhone has become its fastest selling device, helping the U.K. division of Telefonica (TEF) achieve its strongest quarter on record. Sales were up 9.5 percent in the last quarter of 2007 — the first to show the effects of the iPhone. O2 did not provide sales figures for the iPhone alone, but it did say that the device helped add 483,000 customers and 276,000 contracts to its roster.

According to O2, the iPhone has the highest satisfaction rating and the lowest return rate of any phone in its lineup. Moreover, iPhone users generate roughly 30 percent more revenue per user than the carrier’s average customer.

O2 also announced today that starting March 14, it will be offering the iPhone for sale in Ireland — only the fifth country authorized to carry the phone (after the U.S., U.K., France and Germany). For pricing details, see the Irish Times’ report here.

In remarks yesterday at a Goldman Sachs technology symposium, Apple COO Tim Cook said his company will be cutting more deals in both Europe and Asia to reach its sales target of 10 million iPhones in 2008. See Apple’s COO calms the waters.