M option with dial command

When i make Dial(SIP/703,15,tTrm(mess_dsoft)), the music is default class from music on hold file, not mess_dsoft class. If i make SetMusicOnHold(mess_dsoft) before the dial, it’s ok for the calling, but if i take another line, the caller have the mess_dsoft for music on hold, and i want he have the default. rm(class) don’t work ??? i’m mistake ???

Thank’s for your help