GSM clock and phones

Hello everybody When launching a local BTS (like with openbts on usrp or similar), one needs to feed the BTS with a precise clock or a reference clock, depending on the hardware. This step is needed because phones need a precise clock from the BTS, otherwise they don't camp. Is that correct? If yes, my question is: how can a phone determine that the clock given by the network is precise enough? I'm a bit confused: how can the phone say that's not good, if it doesn't have a precise clock itself? I tried to search something on google but results got are completely out of scope. Can someone share some link? Thanks to all Dario.