Here are my 2 cents on the issue (please correct me if I am wrong) .. I will try discussing it in context of a PLMN
I think once VMSC sends release message to GMSC with the casue being no answer, then the GMSC must connect to IVR and would send an ANM to VMSCA so that VMSCA tells A party that it connects to a voice channel .. otherwise party A will not be able to listen to announcement.
What do u guys think??