The voicecall-ui displays a toggle that enables you to select the corresponding audio device in case a bluetooth device is connected and detected as supported.
If that is not the case for you, then try to remove the pairaing with the corresponding bluetooth speaker, re-pair them again and set a proper device type in the settings. This should do the trick, since the latest two SFOS updates improved the audio handling of connected Bluetooth devices (see Unable to route phone call audio to bluetooth headset for more details).