AT&T today said it has finished upgrading its network in New York City, improving wireless service where densely-packed iPhone users have caused dropped calls, slow downloads and other issues.

The Dallas, Texas-based company said the project, which expanded the capacity of its 3G network, has steadily improved service quality in the boroughs of Manhattan, Bronx, Brooklyn and Queens over the last six months.

In February, AT&T said that upgrading its infrastructure, particularly in New York and San Francisco, was a top priority. Similar improvements in San Francisco are still a work in progress, AT&T spokesman Mark Siegel said.

“We’re working to make sure our customers have a great experience every time they make a call, check email, download a song or video, or surf the Internet on their AT&T device,” said Tom DeVito, AT&T’s general manger for New York and New Jersey. “Adding more capacity to cell sites is just one way we are doing to that.”

Earlier this week, Apple announced that it has sold over 1.7 million units of its new iPhone 4 in the first three days. The iPhone 4 went on sale Thursday in the U.S., Britain, France, Germany and Japan.