AT&T is investing heavily in the Nokia Lumia 900 and spending more money on the Windows Phone handset launch than it did on the iPhone, according to a report in Ad Age.

The wireless carrier may be spending as much as US$150 million to advertise the phone, which is an AT&T exclusive. This is a lot of money, even for a carrier like AT&T and leaves everyone wondering why it would spend so much on a platform that is losing market share.

Ad Age claims that, after years of exceptional sales, AT&T's smartphone growth is slowing now that the iPhone is available on Verizon and Sprint. Rather than rely only on the iPhone alone, AT&T is looking for another exclusive, blockbuster phone that'll boost its subscribers. Right now, according to Ad Age, that phone is the flashy Nokia Lumia 900.

Update: A report from The Verge calls into question the $150 million figure from Ad Age. Sources familiar with the matter say the actual dollar amount is much lower than $150 million and more in line with any high-profile phone launch.

The Verge also points out that Ad Age uses a comScore report to claim that AT&T's smartphone penetration is slowing. A look at the quarterly earnings shows that Verizon's smartphone ownership grew 5 percent between Q3 2011 and Q4 2011, while AT&T increased a very close 4.2 percent.

*Verizon is currently in the process of acquiring AOL, Engadget's parent company. However, Engadget maintains full editorial control, and Verizon will have to pry it from our cold, dead hands.

This article was originally published on Tuaw.