Grandstream Networks UCM6510 IP PBX User Manual page 196

Note:
In case the users have deleted the system MOH files, there are two ways to recover.
1. Users could download the MOH file from this link:
http://downloads.asterisk.org/pub/telephony/sounds/releases/asterisk-moh-opsound-wav-2.03.tar.gz
After downloading, unzip the pack and upload the music files to the UCM6510.
2. Factory reset could also recover the MOH file on the UCM.
Firmware Version 1.0.1.12 UCM6510 IP PBX User Manual Page 194 of 277