I’d think this is more switching problem in Core than problem with MO or MT sides.
Core has to switch to calls to MSCs and it might be possible that one of the routes aren’t correctly defined so in those cases the call doesn’t connect properly.
Also likely that higher layers treat this error as destination unavailable or out of order.
You can ask core guys failure rate per MSC. If failures are due to specific region or pool of MSCs then there will be your answer.
Cause No. 27 “destination out of order” (UMTS specific cause values for call control) This cause indicates that the destination indicated by the mobile station cannot be reached because the interface to the destination is not functioning correctly. The term “not functioning correctly” indicates that a signalling message was unable to be delivered to the remote user; e.g., a physical layer or data link layer failure at the remote user, user equipment off-line, etc.