First of all there is no relationship between SDCCH_ABIS_FAIL_CALL and TCH drop rate. This counter is encountered during SDCCH setup procedure.
The Probable Causes for SDCCH_ABIS_FAIL_CALL are:
1)Ghost Random accesses
2)Mobile in Congestion area or the Mobile is in weak coverage area leading to double reservations: RACH repetition
3)BCCH frequency-BSIC reuse
4)bursts of random accesses (HO access)
5)”channel activation not acknowledgement” message failure