Your Wish:

"Share the POTS port on my ISDN modem"

TSS Telephone Sharing Switch (Various Models)

Shares 1-6 TelCo lines between up to 64 desktop modems. Oriented toward dial out applications.

TSS-2PI Two-Port Telephone Line Sharing with Outbound Priority

A two-port telephone sharing switch that can interrupt a call in progress and give the line to the priority device.