Hello,
in fact, ACM with cause code and in-band info can be sent at the same time – pls see Q764 – 2.2.4 Tones and announcements. As far as i understand – it have to work as following – originating PSTN must generate statistic record about failed call due B-BUSY but does not disconnect trunk in order a-subscriber listens announcement. Then the circuit is released after REL with CC=31