Thanks to the success of Apple's iPhone, AT&T's customer base has increased over the past three years. However, the network has also been pushed to the breaking point.  In terms of data speed, AT&T appears to be the fastest network in the U.S., but its voice network has been plagued with reports of dropped calls and poor voice call quality.

However, starting this year AT&T has been beefing up its network, investing an additional $2B USD in infrastructure.  Company Chief Executive Officer Randall Stephenson said in a CNBC television interview which aired Tuesday that those efforts are paying off.

He said his company is close to bringing its mobile voice call quality close to where it wants it.  He states, "We've been going hard at the voice quality issue."

Stephenson did not discuss dropped calls, but AT&T spokespeople have expressed to us previously this year that the company believes dropped call rates are falling.  Those claims are at odds, though with a recent study that claims dropped call rates are actually going up.

Its voice network has reportedly had dropped call rates as high as 30 percent in recent years (AT&T's own statements have indicated a much lower rate of < 5 percent of calls dropped, but AT&T has continually been unable to provide us with dropped call rate studies that focused exclusively on urban areas).

AT&T is bracing itself for a massive surge in subscribers with the release of the iPhone 4. Hopefully the investments in its wireless network infrastructure will show some real world benefits once these phones start reaching customers later this month.