Using zoom on Android with Bluetooth speaker

Whilst my android devices work perfectly well with a Bluetooth speaker, this connection no longer works once the device enters a zoom call (neither as host or participant). The little button on the top left that suggests switching speaker between phone and Bluetooth, actually switches between phone internal (i.e. ear piece) speaker and external speaker, but no Bluetooth. Once I exit zoom, the phone resumes it's sending of sound to the Bluetooth speaker. I've tried this with an LG G6 and a 2020 Motorola edge, same response. Question: how do I get my Android device to send sound to a Bluetooth speaker while in a zoom call? Thanks