Analysts are mulling over Apple's iPhone sales numbers and are alarmed at a perceived discrepancy in the figures.

Apple claims slightly over 3.7 million iPhones were sold in 2007 -- yet AT&T this week revealed it ended the year with "just at or sightly under 2 million iPhone customers."

That 2 million has been boosted somewhat by an estimated 300,000 to 400,000 sales in Europe, analysts believe.

The discrepancy is that the 3.7 million iPhones Apple says it has sold and the estimated 2.4 million sold by its network partners still leaves 1.3 million of the devices unaccounted for.

That implies that around one in three iPhones are being purchased in order to unlock the device for use on other networks and/or for use with unapproved third-party applications.

While it's possible some iPhones were sold over the Christmas period but not activated immediately as new users (perhaps) worked to cancel their existing mobile contracts, the discrepancy still implies an active market for unlocked iPhones.

Apple chief operating officer Tim Cook told analysts this week that the company believes the number of unlocked iPhones in the wild to be "significant," but declined to furnish accurate figures.

Toni Sacconaghi of Bernstein Research cites these figures in order to claim a substantial market in unlocked iPhones, and speculates this news may also mean a build-up in iPhone inventory.

"It indicates end-user demand for iPhone is lower than many investors may think based on Apple's sales figure -- and it points to slower iPhone sales in the current quarter, since much of this inventory is likely to be drawn down," the analyst explains.

Macworld.co.uk is an InfoWorld affiliate.