DEAR
i think your question’s answer is related to frequency planning
what i mean is if u have your carriers from 1 to 20 and u have from 1 to 10 as BCCH carriers so from 11 to 20 are TCH carriers and when u make TCH frequency plan follow the BCCH frequency group u can find call drops even when u use TCH carrier because u use the same plan for BCCH carriers and TCH carriers
Best regards