Hey there, lulu206.
After reviewing your post, it appears that the volume level of your iPhone 6s isn't loud enough when making or taking calls after pairing it with your 2016 Lexus. I'm happy to help.
Get help connecting your iPhone, iPad, or iPod touch with your car stereo -- Follow the steps in this support article.
If the steps in this article haven't resolved the issue, please respond to the following questions.
- What is the model of your Lexus?
- Is the volume level acceptable for other features?
- What iOS is your iPhone SE currently on? (Settings > General > About > Version)
Let me know how that goes.
Take care!