Its not that simple, you'd think it would be, each earpiece has a switch (PTT) and earpiece (little speaker) but the manufactures actually tend to put a bit more circuitry in there for some reason. Also, the impedance (resistance - sort of) of each speaker is different.
So you need to get one for the sort of radio you're using. At one time there was a company that manufactured a PTT and earpiece which had 3 different adaptors for the main types of radio, but you'll not get one that converts from one brand to another.
