Plugging microphone cable into combined mic/headphone jack

I have a Dell Inspiron 14z with a combined mic/headphone jack and Ubuntu 15.04. I have an old headset (temporarily) that has separate mic and headphone plugs. On a friend's computer running Windows, I could plug the microphone plug into the combined jack and use the microphone on the headset. When I try to do the same thing on Ubuntu, sound settings does not seem to recognise the microphone. Plugging in just the headphone plug allows me to use the headphones part of the headset as expected, without further configuration.

Is it possible to get this to work? If so, any tips would be appreciated.

(I realise I could go and buy a hardware combiner, but it would be good to make it work without one if that is possible.)