"Leeching" clock information from cell towers
Heh, very precise timing "on the order of 10us" can only be given to you by an Internet connection through which you then connect to the closest NTP server you can find. The GSM functionality for broadcasting time data is called NITZ and according to its Wikipedia article it's anything but precise (or reliable). Moreover you can't "leech" this data off of any BTS because the very reference specs of the NITZ standard (published by ETSI, the GSM standardization body) say that the data is obtainable only "upon registering on the network" (if you don't believe it, check out also this, this or even this). Seriously just get a GSM module and put a SIM card with some cheap plan into it instead.