That means it takes longer for users to send pictures, video and other content from the phone.
It's the latest glitch to hit the iPhone 4, which went on sale in the U.S. two weeks ago. Last week, Apple acknowledged that holding the phone in a certain way might reduce its wireless performance, and that all iPhones show the wrong signal strength in some situations.
Dallas-based AT&T said Wednesday that a software glitch in network equipment made by Alcatel-Lucent is to blame for limiting the upload speeds of the iPhone 4. AT&T says a fix is in the works.
Alcatel-Lucent spokeswoman Denise Panyik-Dale confirmed that there's a software problem in the company's cellular base station equipment.
Because Alcatel-Lucent isn't the only supplier of base station equipment to AT&T, the problem only affects some areas. AT&T spokesman Mark Siegel said the flaw affects 2 percent of the phone company's customer base.
The iPhone 4 is the first phone designed to take full advantage of an upload-speeding technology that AT&T put in place in 2008. In theory, it's capable of uploads that are 10 times faster than those of the previous iPhone model.
But where the network is affected by the software glitch, some users report the new phone is getting upload speeds limited to 100 kilobits per second, somewhat lower than the speeds usually achieved with the previous iPhone model.
In its statement (quoted by Engadget,, AT&T said the following:
"AT&T and Alcatel-Lucent jointly identified a software defect -- triggered under certain conditions - that impacted uplink performance for Laptop Connect and smartphone customers using 3G HSUPA-capable wireless devices in markets with Alcatel-Lucent equipment. This impacts less than two percent of our wireless customer base. While Alcatel-Lucent develops the appropriate software fix, we are providing normal 3G uplink speeds and consistent performance for affected customers with HSUPA-capable devices."