How to convert from an EasyLyte RJ-12 connector to and RS-232 serial connector

I have a device with an RJ-12 connector, I need connect to a computer by RS-232. This is document of device:

Computer Connection:

The EasyLyte systems can be interfaced with an external computer by means of an RS-232 serial interface. Potential users will require the following information:

EasyLyte RS-232 Protocol

Transmission Rate 1200 baud
Bit Configuration 8 data bits, 1 stop bit, no parity

This interface requires a serial cable from Medica. A telephone-style connector at one end of this cable plugs into the receptacle labeled on the back of the EasyLyte. The other end of the cable is a female D-Sub 9 pin connector with the following pinouts:

Pin #2 TxD

Pin #3 RxD

Pin #7 CTS

Pin #8 RTS

Pin #5 Signal Ground

RS-232 has 9 pin, RJ-12 has 6 pin. I don’t understand this field so I don’t know which pin of RJ-12 is connected to which pin of RS-232?