Causes for tch_abis_fail are:
1.Missing acknowledgement of channel activation.
2.Missing indication of call establishment.
3.Receipt of an error indication, corrupted messages, missing measurement results from the BTS.
4.When the timer T3109 expires in a BSC in call release phase while waiting for the Release Indication.
What needs to check.
2.LAPD Overload of cells.
3.Signalling Load of the BSC.
4.BER in TX path(ZDMX).
This counter is NSN specific only.
In general,this counter is usually subtracted from the total attempts in the denominator as mostly it peggs due to T3109 expiry(pls check your TCH Drop formulae).If its very high for a cell or BSC,you need to check step by step all the mentioned points.