pix has already explained when TCH Assignment Failures occure:
“TCH assignment is the phase during call setup when the BSS wants to shift the MS from SDCCH to TCH.
The TCH assignment is divided in 2 parts :
1- the BSS checks that a timeslot TCH is available in the serving cell (or in neighbor cells, with directed retry)
2- the BSS asks the MS to move onto the TCH timeslot and to send an initial confirmation message while on this TCH timeslot.
In part 1 : there is a TCH REQUEST, which leads to a TCH ALLOCATED (= available TCH) or a TCH REJECTED (= no TCH availble)
In part 2 : the MS will try to access this TCH. If the dialog BTS/MS onto this TCH is possible, this is a TCH SUCCESS. If the dialog is not possible, this is a TCH FAILURE.
The TCH CONGESTION occurs only in phase 1.
The TCH FAILURE occurs only in phase 2 (with most common problems such as RADIO or HARDWARE PROBLEMS)”
For example, if you have a cell with BCCH on P-GSM band and TCH on E-GSM band, and you make a call via repeater that doesn’t support E-GSM band. During call setup, you get SDCCH on P-GSM frequency, and BSS gives you a TCH on E-GSM frequency. Since repeater doesn’t support E-GSM, you won’t be able to continue communication on allocated TCH (since you have to low RxLev on E-GSM TCH, for example RxLevBCCH(P-GSM)=-90 dBm and RxLevTCH(E-GSM)=-115 dBm). If TRX is faulty, you could get the same message. Probably very strong interference on that TCH would lead to the same.