MOS scores and R factors serve a similar purpose - they provide a holistic "index" that represents the overall quality of the call. MOS (Mean Opinion Score) is a 1-5 range indication of the perceptual quality of the call. R factors are 0-120 "transmission quality ratings" that are also indicative of the quality of the call.
So why have both?
MOS scores are generally accepted as representing the user's opinion of the call - with 4-5 representing a typical good-excellent range. MOS is however a relative range (which is confusing given that an "Absolute Category Rating" test really gives a relative result). A narrowband (8kHz) call will have a MOS in the 1-5 range however a wideband (16kHz) call will also have a MOS in the same range - and a better sounding wideband call could have a lower MOS than a narrowband call - confusing!!! The reason for this is that a narrowband call's quality is given in comparison to other narrowband calls and a wideband call in relation to other wideband calls. It is therefore important to say "this call had a MOS of 3.9 and it was a wideband call".
R factors have a range that is continuous through narrow and wideband - narrowband range is 0-95 and wideband 0-120 or so. This is less confusing and does not require the added qualification of the R factor being narrow or wideband.