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).