According to AT&T, the company will increase its cell sites by 25 to 35 percent in San Francisco and New York, , and by 35 to 45 percent in Chicago. The expansion will hopefully increase coverage in buildings and allow the network to be more flexible in the case of . The carrier says that in most markets, these increases are equivalent to several years of building new cell sites.

"It's pretty good news, assuming it actually goes through," said Avi Greengart, research director for consumer devices at market research firm Current Analysis. "If you're an existing AT&T iPhone customer, particularly in New York or San Francisco, there should be a lot of additional cell phone capacity that comes online because of this deal. It's gotten better, but there are still a lot of devices in a small area. That may alleviate a lot of the problems people have in those two locales."

However, certain adjustments will have to be made to existing cell sites. Currently, while T-Mobile and AT&T use the same 3G technology--High-Speed Packet Access (HSPA)--the two networks rely on different wireless frequencies, making their handsets incompatible. (They do, however, share a frequency for the older EDGE technology which allows for voice calls and slower data transfers).

AT&T has said that T-Mobile's cell sites will be gradually updated to support AT&T's frequencies as well. That has the added benefit of eventually freeing up the slice of wireless spectrum used by T-Mobile's 3G service, a valuable asset as the radio frequencies available to wireless providers have become few and far between.

"This should translate eventually into a better user experience," Forrester Research's Golvin said. "T-Mobile has several spectrum assets, some of which will be used to help AT&T build out its next-generation LTE network."