The Cisco SX20 does not normally accept 3rd party microphones.
With an adaptor from the 4pol. 3,5mm you can use any microphone you like (using a phantompower-injector if needed), or connect a mic-level signal from a DSP. By using one of the Mic-connectors instead of Line-in uses the codec echo-cancelation.
T - Mic+
R1- Mic -
R2- Resistor return to codec
S - Sh
Alternative, if you need a secondary toggleswitch for mute, connect two resistors, R100 & R360 i series between S and R2, and the toggleswitch parallel to R100.
Momentarily bypassing the 100 ohm resistor, lowers the resistanse between Sleeve and R2 from 460 ohm to 360ohm, makes the codec to toggle mute/unmute.
Note, there is not sufficient phantompower to drive an condencermic. You must use an external phantompower-injector.