I just did a diagnostic: 808650862

Fastest way to get a reaction on a diagnostic is to call Sonos about it.

It sounds a bit like there is something on the phone blocking the ‘discovery’ of your Sonos devices on the local network, what type of phone make/model/OS version is it? Do you have any security software running on the phone, such as a VPN service, firewall or antivirus etc?

Any MAC address ‘spoofing’ features running on within the network connection properties, such as ‘Private WiFi Address’ (usually enabled by default on Apple iOS mobiles), or maybe ‘WiFi calling’ is getting in the way of things?

Also ensure QoS and/or Airtime Fairness is not running on the local router …and (if applicable) that IGMP Snooping and Multicasting/Broadcasting is enabled across both the LAN and Wireless LAN segments (most routers enable this be default, but not all so check your router user-manual). 

If using managed switches on the local network, then you may also find these links helpful…

https://support.sonos.com/en-us/article/configure-stp-settings-to-work-with-sonos

There might also be an issue of device discovery arising from Wireless interference, in which case also see this Sonos Support article…

https://support.sonos.com/en-us/article/reduce-wireless-interference

Also see..

Wireless Interference Video:

Wireless Interference and Sonos

Hope the above information helps you to resolve the matter. If the problems persist then it maybe best to speak to Sonos Support Staff, together with a diagnostic report, to see if they can perhaps  assist you to resolve the local ‘device discovery’ issue.