One source of confusion is that there are two different types of switching jacks, as well as plain non-switching jacks.
The stock jack does not switch the speakers off when a headphone is plugged in.
I found only one readily available jack that could to that, and a few people have converted to that jack. You might want to see if that's what you have, or if that's what you want to have. This jack is the KobiConn 161-0083 (or KobiConn 161-0083-E); I have posted instructions for the conversion and can do so again if anyone needs them.
Many other jacks have switches and are called switching jacks, but their switches are not configured in a way that allows the 120-ohm series resistor to be used with headphones and bypassed when speakers are used.