Yes, the caller should hear the announcement and the phones called should ring in the same time.
If not the caller will have to wait the end of the announcement + time to pickup a phone during ringing time = too long.
If nobody pick up the schedule is :
Annoucement (15 sec) + ring tone (30 sec) = 30 sec
"all our lines are busy" (15 sec)
voice mail with no mesg
if somebody pick up before 15 sec, the caller will even not hear ring tones.