If you look at a modem, with it's small central processing unit and special-purpose digital signal processor, and then look at a modern PC, with it's large CPU and general-purpose DSP on the sound card, you may wonder if the hardware duplication of an external modem is necessary.
A "WinModem" incorporates the CPU and DSP of the modem into the slightly-enhanced fabric of a PC. They are called "WinModems" because they originally only shipped with Microsoft Windows device drivers. These device drivers presented the illusion of a serial port attached to a Hayes AT-style modem. For a long time only Windows versions of these drivers where available. Some manufacturers now provide Linux versions of their device drivers as well, these modems are jokingly called "LinModems".
It is probably possible to use a LinModem as a Linux console. At the most this would require altering the source code to dumb-down the AT command emulation of the modem and recompiling the kernel.
Boot loaders, however, work in a very confined software environment and struggle to support a simple serial chip. Considering that some boot loaders do not even handle interrupts, handling the complex DSP of a LinModem is well beyond what is practical.