Geschafft:
exten => 2,1,Playback(auswahl2)
exten => 2,2,ExecIf($[ ${DEVICE_STATE(SIP/$**600@Fritzbox)} = BUSY ]?GoTo(2,4):Playback(hello))
exten => 2,3,GoToIf($[ ${DEVICE_STATE(SIP/$**600@Fritzbox)} = BUSY ]?:2,2)
exten => 2,4,Dial(SIP/**620@Fritzbox)