Despite efforts to prevent iPhone-cracking, AT&T and Apple appear to be losing the battle to keep customers locked into exclusive partnership.

Since Apple and AT&T started reporting the number of iPhones sold versus those operating on AT&T's mobile network, the gap between the figures has consistently grown.

During the first weekend of iPhone sales, the gap was 124,000 units. At the end of the third quarter of the calendar year, it had grown to 300,000 iPhones. Last week, Apple and AT&T revealed that gap had increased by a factor of five in the fourth quarter, to 1.7 million units.

Meanwhile, according to research firm, RBC Capital Markets research, the total number of units sold currently stands at 3.7 million units across the US and Europe, leaving the 1.7 million phones representing a 45 percent unexplained gap.

Only part of the 1.7 million unaccounted for phones can be explained by sales through European carriers such as O2, Orange and T-Mobile, which currently stand at 350,000, leaving a total of 1.35 million phones unaccounted for. If Apple's chief operating officer, Tim Cook, is correct, the figure should be at 20 percent of the total number sold -- 750,000. However RBC Capital Markets estimate of 30 percent is closer to the figures reported in Apple and AT&T's reports -- 1.3 million across Europe and the US.

Attempting to stem the growing number of iPhones being cracked, Apple has imposed various constraints on access to the iPhone, such as limiting the number of units that can be sold per person and forcing buyers to use their credit cards, to restricting the type of software that can be transferred from the iPhone's flash memory to the main memory.

Liam Tung from ZDNet.com.au contributed to this story.

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