I mean : the call is setup in the cell, and after 15 seconds it is sent (handover) to another cell :
– which target cell exactly ?
– why ? (reason for HO ?)
The reason for long SDCCH is not important : first you must understand what is the problem with the TCH.
Maybe once you fix the TCH issue, you’ll see that the SDCCH is fine also.
Another investigation : do you think it could be a TRX HW issue ? Can you try to lock the TRX that gets most of the TCH allocation, and check if other TRX’s will carry the TCH longer ?