An analog line into a building from a typical phone company is an RJ-11 public switched telephone network two-wire scheme, a tip and ring lead. These lines carry voice or data over the local loop into central office of the regional phone company that supplies the switching equipment, signaling equipment, and batteries for telephone operation, and then on to it's receiving end. Apple's GeoPort is like any other modem on the market today, it conforms to the same electrical standards used by the modem industry enabling it to communicate over public switched lines.
The GeoPort uses only a pair of the four wires. The GeoPort Telecom Adaptor is only wired for pins 2 and 3 (red/green) of the phone connector.
As for the GeoPort working on a multiple-line analog phone line: If the multiple-line analog phone line uses the telephone company standard, four conductor RJ-11 wiring, pins 2 and 3 (red/green) will be the wire pair a GeoPort will use. Pins 1 and 4 (yellow/black) will have to be switched to pins 2 and 3 for operation with a GeoPort Adaptor.