At least in 3GPP networks (ie. GSM/UMTS/LTE, all of which depend on precise 27MHz clock in the handset) the PLL lock to network's time reference gives more precise time reference that you can reliably get from GPS and essentially for free. The MS (ie. your phone) needs to maintain that PLL lock in order to stay associated to the network (which itself has traceable time/frequency calibration to some kind of national frequency standard and GPS clock).
AFAIK in 3GPP2 (ie. US-style CDMA) the handsets actually have GPS receiver that gets used in the initial association phase to speed up the timing PLL lock (which for the initial cell acquisition has to be significantly more precise than in 3GPP systems)
AFAIK in 3GPP2 (ie. US-style CDMA) the handsets actually have GPS receiver that gets used in the initial association phase to speed up the timing PLL lock (which for the initial cell acquisition has to be significantly more precise than in 3GPP systems)