Its been a few years since I did an ISD interface, but the one I did used a
C54 to address into 4 second intervals so I could randomly select and nn
second message (I think you can divide up the memory into 2,4,8,... second
intervals?).

I know you can do it.  The tricky part I found at the time was to RECORD the
intervals and not overlap.  In other words you can set a beginning address
for the ISD but you need to control how long the interval lasts.  Like I
said, its been a few years since I did that job.