Your stated reason for making this change is to get off of the unsupported EOL Cisco devices. But the solution your implementing seems like it's an even more unsupported solution. If Blue Cross is committed to keeping this dail-up solution active, then replace the EOL Cisco devices with the current Cisco replacement.
I'm no Cisco solution expert, but a Cisco 2960 with a PRI interface card will act like a bank of modems (21 modems on each card) and do TDM (PSTN) to serial and seems like it would be a drop-in replacement and fully supported by Cisco.
If you want me to connect you with a Cisco rep who can solution this for you, then please feel free to contact me off-list.
It just seems very ill-advised for Blue Cross to run a critical business system on a hacked-together solution like this. When you're on vacation or moved on to another job, who is going to fix this thing? Ok, I know Adam will but still! ;)
John