|How old is the house or how old is it's phone wiring and as already asked, where is the house? If you're in the US there's probably a black wire there too. It may have been cut back to keep it out of the way so you might not see it.|
I just opened up a 1960's era rotary phone and it used all 4 wires. It wasn't obvious what each of their functions were as they connected to a single block inside the phone.
It seems like I recall that since modern phones only need 2 wires to function and older homes already had the 4-wire cables then the other 2 wires were used if the customer wanted a second line. That is, the original intent of running a 4-wire cable wasn't to accomodate 2 lines but with newer technology that became possible and older homes didn't need to be completely rewired.
The older rotary phones had those big 4-prong wall plugs so when the newer phones came out using the modular RJ11 plugs the phone company often would come out and rewire the wall plugs anyway.