"The 4 million phones sold in the first six months is also double the initial run rate of the
Razr," says Abramsky. "From our perspective, the early performance of the iPhone is nothing short of remarkable relative to other historic phone launches."
Analysts and investors also have been concerned about the difference between the number of iPhones sold by Apple and the devices activations registered by AT&T.
In its fourth-quarter earnings, AT&T indicated that about 2 million iPhones had been registered for the service on the company's network while Apple sold about 3.7 million iPhones during that quarter.
The difference of 1.7 million phones could indicate some excess inventory in the channel, which could come back to bite Apple's target of 10 million for the year and stoke fears of lower-than-anticipated demand for the phones.
Apple has not commented about the unlocked phones yet, and the company did not respond to a request for comment for this story.
Apple could have sold about 400,000 phones in Europe during the quarter, and about 20% have been sold unlocked, estimates Keith Bachman, an analyst with BMO Capital. In the U.S., about 25% of the phones sold could be unlocked.
The rest are likely to still be in inventory, while some will be returned and a small percentage are being used for nonphone features, such as iPod touches, until the owners can switch their wireless contracts to AT&T, says Gottheil.
The high percentage of unlocked iPhones should not deter investors, says Abramsky. "It's a leading indicator of the demand," he says.
As Apple signs up contracts with carriers in countries such as Canada, Italy, Spain, and Australia, Apple could bring more users into the fold, he says.
Meanwhile, U.K. mobile company O2 is doing its part to make the iPhone more attractive to its customers, in what some see as an attempt to revive flagging sales. The company said that while the cost of the iPhone remains unchanged, it will give three times as many free calls and text messages on its £35 ($69)-a-month service plan for iPhone customers.