Hello nachi,
you may detect Phantom RACH from Timing Advance measurments since you will notice samples from impossible distances.
ex: i made a drive test for a cell that can barely cover 5 KM, and I had High SDCCH assignment failure, from timing advance measurments it reported many samples from 20Km and 26Km which was impossible! therefore increasing RxLEvAccessMin from -108 to -106 made a lot of improvement in the sdcch assignment success rate (from 40% to 10%)