After Apple's developer conference last month anticipation has been mounting for the company's iPhone 5, but the buzz may be slowing down sales of its current offerings, analyst say.

Wall Street anticipates Apple selling nearly 17 million of its venerable iPhone 4 phones this quarter, but new analysis is indicating the actual number could be below this consensus.

Sanford Bernstein's Toni Sacconaghi says the Street is not taking into account the slowdown due to the upcoming iPhone 5.

Sacconaghi sees a wide range of possible unit volume numbers for that fiscal Q4, from 13.1 million units to 20.7 million units, depending a lot on timing of the iPhone 5.

In the best case, the iPhone launches in the first weekend of September and iPhone sales until then decline only 15 percent quarter for quarter, he said.

The launch weekend could see 3M iPhone sales (compared to 1.7M last year, when the iPhone launched in 5 countries that accounted for a then estimated ~60 percent of iPhone sales), given the boost from Verizon this year, where we expect the iPhone to launch simultaneously.

We expect post-launch weekend run-rate to drop by half and then continue to drop until it stabilizes at 20 percent global smartphone share at the end of the quarter.

Sacconaghi believes it will be important for Apple to expand its presence with Asian carriers, and to offer a lower-priced iPhone model, which he thinks will happen sometime in the next couple of years, as the growth at the high-end of the smartphone market won't be as strong as at the low-end.