It has the 2.5mm connection. I plugged it into my phone and tried to make a test call. The volume through the headset earpice is really really quiet. I turned up the volume and even went into the admin settings and turned up the gain, but it's still really quiet. I can hear fine on the other line when I'm talking.
I tried it in a different SPA942 and I'm still having the same problem. I looked online and the SPA942 doesn't require an amplifier or anything.
So then I'm just wondering if I just got the wrong thing. I looked online and this one was specifically recommended for the SPA942: