Jefferies iPhone chartA new report from Jefferies suggests that Verizon and AT&T both regularly see a drop in phone sales just before and after a new iPhone launches. The trends are clearly visible in the chart above. Note the sharp decline in sales before the iPhone 3G launch on July 11th, 2008, for example, and the decline shortly after during the fourth quarter of 2008.

The firm also found that net customer adds have been markedly lower for U.S. carriers in the wake of an iPhone launch. The first quarter of this year marked the worst quarter in years for net customer adds, Jefferies said.

Of course, it’s possible that a lack of new smartphones could be one reason that sales drop off after iPhone launches. The only notable flagship competition in Q1 would have been the Galaxy Nexus, but that was only released on Verizon. But it’s also likely that sales drop because customers wait with pent up anticipation before every iPhone launch. Naturally, sales spike when it’s released, and then drop again. We’re guessing Jefferies will see yet another peak when Apple’s next iPhone is announced later this year.

[via All Things D]