A direct cable connection
from computer to computer.
Usually, modems operate on the RS232 standard.
In that standard, one line is for transmit and one line is for receive. (The only other required line is a common ground or node.)
Cabling two computers together directly, instead of supplying modems and a telephone line, requires a crossover cable so that the transmit and receive lines switch roles. Such cables are called null modems.
The same crossover technique can be used in CAT5 or other twisted-pair Ethernet cables as well. This removes the need for an intermediary hub between the two computers.