Hi mostafa
I faced the similar kind of problem where the dropped call was due to “other reasons”.
I beleive that if call drop is happening due to “other reasons” it doesnt have anything to do with the BSS, and has some relation to the BSC. And i think if you test the TRA SNT’s in the BSCX/TRC you might find something interesting, like i did.
Strictly refering to Ericsson System, even though the TRA’s were functioning fine the RP data for teh SNT was defined wrongly in my case. So see if you get a fault code, if u do let me know maybe i can help further.