[Freeswitch-users] music on hold for opensips users

I am using FS as a media server for my OpenSIPS server. I want to know if I can make opensips' users listen to music via FreeSWITCH, until the opensips callee picks up. Yhe point that I don't get it, is how FS knows when the opensips' callee change from busy state to available. Thanks a lot