It is an adapter cable to adapt an RJ11 jack to a DE-9 connector.
It was most likely used in a specialized situation between two devices, perhaps between a modem and a telephone service jack.
If you have no use for it, cut off the connectors and repurpose the cable for something else.
Agree. the 2,3,5 are the RS32 data TX,RX & GND, the hands shake connections are absent, in some cases they can be jumpered out each end and software HS can be utilized, if needed.