I think PCI bus is 32 bits and ISA is 16.
They are designed for PC motherboards with 16/32-bit processors, megabytes of memory, DMA, multi-master, etc, etc.

This is way overkill for an 8051 embedded 8-bit micro!

I think you would be well advised to stick to an "external" (ie, RS232) modem, or maybe consider a modem chip with an 8-bit bus interface?