I've been doing some research into speakerphone modems. The situation is not as bad as I had thought for finding a replacement for the (seemingly) unoperational Spanish modem.
Speakerphone modems have, besides the two RJ11 sockets, a microphone and speaker socket. They ordinarily use an independent microphone and speakers. But, when used with a soundcard, the modem and soundcard can be interconnected. The mic goes to the modem, and the modem speaker to soundcard line in. So the voicemodem's output goes through the soundcard.
There are quite a number of PCI speakerphone modems on the market. The cheapest are the Winmodems (no onboard processor but use the motherboard CPU to operate the modem in its "spare time".)
On my replacement Box motherboard, I've removed the back-panel mic and speaker sockets and am replacing them with stub 1/8" stereo headphone cables/sockets.
Some modems and soundcards have a "TAD" (Telephone Audio Device) connector. This has ground, mic, left and right speaker lines. I am looking into this further. I'm still not clear if this connector can be used for speakerphone operation, but it may be a way to hook a PCI speakerphone modem to the Dot motherboard without redoing internal wiring.
I need to finish my wiring harness trace I started a while back so I can understand how all the components interact in the Dot...