Customers are slow to upgrade devices, but AT&T is still gaining plenty of new subscribers.Coming a few days after a mixed earnings report from rival Verizon Communications, AT&T's (T 5.22%) second-quarter report suffered from a similar problem. Wireless customers have been slow to upgrade their smartphones, leading to lower equipment revenue for the wireless leaders.AT&T saw equipment revenue drop 5.8% year over year in the second quarter, enough to cause the company to miss analyst expectations. But unlik ...