For assistance connecting the Plantronics CS540 DECT headset to your snom phone using the EHS adaptor, please use the setup guides that are available in the snom wiki.
n.b. the instructions refer to an RJ4P4C cable. This is not included with the snom EHS adaptor. Instead you should use the telephone interface adaptor that comes with the Plantronics CS540, which looks like this:
The snom EHS adaptor comes with several cables, and you will need to select the correct one for your device. If you find that you have reduced functionality whilst using your headset (for example, you can answer calls but can't hear the other party), check that you are using the correct cable.