Hi Wajid,
I think as you are using an automatic voice mail to perform your drive test”, there is a timer in the Core Network which automatically release the call after 20 minutes exactly.
You can perform several test in static under good rf condition calling this “voicemail” and confirm the drop occurs exactly 20 minutes after your start.
“national call in the network are free, if the duration of calls exceed 20 minutes ,the MSC drops the call.
can i relate this reason for lost TCH and SDCCH radio link.
please advise”
Regers
Testa