Help: I can't simultaneously listen to Spotify on Sonos and my phone

[ojb2]

I have a spotify family subscription, which I use to listen to music and podcasts via a Sonos One and on my phone, an iphone 5. I am in the UK.

We have three accounts on the subscription (me, my wife and my son), and I want to be able to listen to spotify on my phone, while other members of my family can also listen to spotify via the Sonos. At the moment, however, when anyone listens to spotify on the Sonos it immediately cuts out on the phone even if I have changed the account that the Sonos app is logged into. 

I specifically bought a family subscription to stop this happening, but so far without luck.  This is extremely irritating. Could you please explain what I need to do to get around this problem?

Thanks!

Oliver