1. The BTS sends the value of the TA to the MS. The MS doesn’t decide anything !
2. The timing advance is computed in symbols, not in timeslots. 1 symbol is extremely short ! (don’t have the value in mind).
3. The MS is sending 1 burst of compressed voice every 4.615 ms. So there is obviously a buffer in the mobile. More than that, there is a whole encoding, modulating, ciphering, etc… chain within the MS. 1 burst contains the equivalent of 4.6ms of voice (approx).
The TA will just tell him exactly when to send it, compared to the “time reference” provided by the BTS.