I think all you need to do is Airplay from your phone to the speaker.

https://www.lifewire.com/enable-airplay-for-iphone-ios-6-2438634

Thank you John but the speaker does not appear when I press on the airplay icon...

Not sure what purpose ‘group non AirPlay products’ would be in this situation, the Sonos One is an AirPlay 2 product. In general, John’s suggestion should work. 

If you’re not seeing the Sonos One (and not a PLAY:1) as an AirPlay 2 target, I’d suggest a refresh of your network, by unplugging all Sonos devices from power, then rebooting your router while they’re unplugged. Once the router comes back up, plug back in the Sonos products, and see if they now appear as valid targets for AirPlay 2.

thanks for the answer. What’s weird is that it’s working on my Mac, I can see the Sonos One speaker. It’s only on my iPhone.

That would tend to make me think that you’re using a mesh network of some type, and your Mac and Sonos are on one subnet, and your phone is on another. 

If so, you’re probably better off running in ‘wired’ mode. Read through the wired and wireless modes FAQ.