Hi Pary,
Well, MS message (error_in_connection) is generated by MSC rejecting your CM_SER_REQ message. It is internal and cannot be construed as RACH drop as RACH access is done by that time. Secondly its not even AGCH drop because the MS has not allowed Access_grant.
This problem is caused by some problem in the PCM trunks towards the MSC. Majorly it is solved by troubleshooting these PCM trunks.
Best regards,
Garry !!