I’ve never thought the central button (I’ve got the Fairphone headset) could be used for that, I’ll try thanks.
However, using buttons on Fairphone’s earphones is not easy at all. Sometimes even raising up or down the volume is hard to do. Buttons are very small and feeling them under the fingers is quite difficult and you never know where you’re pressing.
However, before buying the FP3 I had an ASUS Zenfone 3. When you plug your earphones in it an icon appears on the top bar. This means it can realize earphones have been plugged (FP3 doesn’t show any icon instead).
All the suggestion I’ve found up to know refers to something in the phone app settings.
The standard phone app that comes with FP3 doesn’t have it (or I still haven’t found it).
I didn’t look for an app to do that. I try to limit as much as possible third party app installation (everybody has got his mental illness, this is one of mine ).
I don’t know if Fairphone OS allows this, but in /e/OS on the FP3, I can edit which status bar icons are shown. By default, the headset icon was not shown in /e/OS either, but I could activate this through the “Status bar” settings (Settings > System > Status bar).