On the computer desk in the living room is an acoustic coupler modem, which requires a telephone handset be physically attached to the top of the modem to make a data connection. But the telephone sitting next to the modem is not the correct type to interface with this type of modem.
Charles Kremp mentions that he and Murray have been neighbors for 20 years, but in an episode of season 1 the Kremps are introduced as the new family that moved in across the street. Meaning the two families have only known each other for a few years.