GSM clock and phones

[Dario Lombardo]

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.