Well, digital net (GSM, 3G) cell phones were simply not around when the current bread and butter airliners were certified, so there was nothing about them in any certification test.
If you want to certify on board cell phone service with a pico cell or WLAN you have to prove that there are no interferences with the onboard system, especially navigation and flight control systems. And indeed some series of the 737NG display system did malfunction with an onboard WLAN network.