Share your experience!
When I make phone call, the speaker is always on. How can I make speaker default to off?
Hi @ThomasMao, sorry to hear about this.
Is this something you have started to experience recently, for example after updating your device or updating/installing a new application?
Are you able to turn the speaker function off during the call? You can turn it off by tapping on "Audio output" on the call screen during a call.
The call should not be in speaker mode as default, so I'd recommend that you reboot your phone in safe mode and see if you experience the same behaviour when third party applications aren't loaded.
Let me know how it went!
You're welcome! 
Is it working only in safe mode or is it working when you rebooted the phone back to the "normal" mode again as well?
If it is only working in safe mode, one of your downloaded applications is the cause of this so make sure that they are updated to to the latest version in the Google Play store!
If you find which is application is the cause of this I would be very thankful if you could get back to me with this information! 
Have a great day!