Why can't I switch to Bluetooth output when calling?

I have a OnePlus 3T and a bluetooth stick in my car. Without any problems I am able to listen to music, but whenever I want to call it doesn't work via Bluetooth. While calling I can switch to the speaker and there's also the possibility to switch to Bluetooth, however when switching to Bluetooth it automatically switches back to the normal calling mode.
How is this possible?

Welcome to Android Central! Go to Settings>Bluetooth, tap the gear icon next to the Bluetooth stick, and see if there is a Phone Audio option. If so, make sure it's checked. If there isn't a Phone Audio option, then you can't route calls through it.

I have HTC Desire 530. Under bluetooth settings I see "Phone audio" option but wont let me click it. Phone is connected to the car and shows contacts. When I try to call it says "check phone". On the dial screen it shows my contact and momentarily shows a bluetooth button. If you are fast enough you can turn it on but only about 2 % of the time. If you get it, the phone connects and bluetooth works. My spouses phone auto connects. Any suggestions on how to get the HTC phone when connected to stay connected in Phone audio mode?

Is it connected to any other Bluetooth devices that are nearby, like a Bluetooth headset? If so, that could be competing for the connection.