AT&T began introducing its 3G network in 2005, upgrading the equipment and antennas at many of its 40,000 cell towers nationwide. It built the network to complement and take advantage of the technology servicing its older 2G or second-generation network. Many phones still use the 2G network, so it must be kept running.

But there are important differences between the 2G and 3G networks, and getting them to work together presents problems, according to engineers who work on the infrastructure.

Take, for instance, the difference in the way voice and data traffic is carried on the two networks. On AT&T’s 2G network, cellphone towers — even ones in close proximity to one another — use different chunks of the radio spectrum to carry information. As phone users move around on foot or in a car, their phones switch from one frequency to another.

Image Disguised or not, cellphone towers often carry a patchwork of network upgrades. Credit... Phil Marino for The New York Times

On the 3G network, all of the cell towers use the same frequency to transmit information. On its face, this would seem to make things simpler. But this technology also adds a wrinkle: when phones get too close to too many 3G towers using the same frequency, they can become overwhelmed with radio noise.

“When you have more than three cell sites overlapping, you get interference,” said one infrastructure engineer who works for AT&T, who asked not to be named so as not to upset the company. “You get bad quality, funky sounds. If you’re doing data, the rates get slower and slower until you lose it.”

Kristin S. Rinne, senior vice president of architecture and planning for AT&T, said the company had done a good job of diminishing the prospect of such interference by limiting the strength of signals from overlapping towers. Moreover, she said, the phones themselves have a role to play; some handsets, she said, do a good job of managing the interference internally, while others do not.