Update: Connecting to the MIPU24 with the "Zoiper" SIP softphone application works fine... audio both ways. So it appears that the MIPU is working ok.

I've got Wireshark captures of both a registration and call from the MVP-130FXS to the MIPU and the Zoiper application to the MIPU if anyone wants to see them. The INVITEs are somewhat different (in more ways than the endpoint IPs and ports) but I'm not familiar enough with SIP at the protocol level to know what may point to the cause of the issue.

I've got a ticket open with Multitech (who make great products), but they're not aware of anyone with history between their MultiVOIP products and Toshiba.