Phone on vibrate/headphones in, why does phone ring out of speaker?

[amacnutt]

I have a Galaxy S6. When my phone is on vibrate, without headphones in, the phone behaves as expected and only vibrates when a call comes in. But if I have headphones in - with phone still on vibrate - and a call comes in, the ringer will sound at full volume through the speaker. Anyone know how to fix this so that it stays just vibrating?

Thanks.