MIPU and FXS are on the same network, no routers, no firewalls, no ACLs between them.

I have not tested the FXS with the MIPU16. I was just mentioning that I had a MIPU16 in slot 0202 that I use with a Toshiba LUCA voicemail system. The MIPU24 is in slot 0205. The only other populated slot in cabinet #2 is slot 0201. So yep, the next slot is left open.