Nachi,
If you are using dualband cells (1 BCCH for both 900 cells and 1800 cells), you shouldn’t have SDCCH set on the 1800 cells.
Mobile located far away, without 1800 coverage, might set their call on the BCCH and then be directed towards the 1800 SDCCH to finish the call setup. The RxLev drops, therefore creating SDCCH drop…
Cheers,
Pix