Hi Pix,
If the MS tries to go to the TCH of the Neighboring cell B from SDCCH of cell A, but due to bad radio conditions is unable to estabilish the connection then it will go back to the cell A SDCCH.
“Unsuccessful Directed retry with reversion to old SDCCH” (T3124 expired (fixed to 320ms)). MS goes to the old SDCCH and reports HO Fail. So SDCCH is not released during that time. It is released only if Timer T8 expires or receives a message that TCH is allocated succesfully. This is the reason to have T8>T3124.
Such problem could increase SDCCH MHT (check value of timer T8 in the BSC)