This cable's pinout scheme differs greatly from the general connection cables used for external modems (here you will need a null-modem, but not an ordinary modem cable). That is why you will either need to buy a cable (and then to check its compliance with the pinouts given below) or make it yourself. Alternatively a cable used to connect two computers via their COM ports will probably suffice. Regular 9-to-25 adapters with the proper cable should still work.

If you require a cable that is terminated with a 9 way connector on your computer then use the following scheme (the numbers refer to the actual pins in the connectors). Be aware that both connectors are 9 way female types.

The following scheme is used for a computer that requires a cable terminated with a 25 way female connector. The figures in the right hand column refer to the 9 way female connector that is connected to the PBX and the numbers in the left hand column refer to the 25 way connector that is connected to your computer.