iPhone 6s Plus is stuck in headphone mode.

Usually, this is a hardware problem. The little mechanical switch inside the jack is stuck. Sometimes, you can get it moving again by inserting and removing headphones several times. If that doesn't work, you would need to have the phone serviced.

Hello,

Try this (it always works for me):

1) Open the Control Center

2) Long press on the music controls

3) Select the output option (little up-arrow with circles)

4) Select iPhone

Good luck.