Seems like the connection between the speakers and the stream, not your iPhones. The controller is merely a remote control, after all.
There’s been some recent reports about issues with BT routers, you may want to contact BT, but I’d start with a simple network refresh. Unplug your Sonos devices from power, then reboot that router. Wait a couple of minutes, then plug back in your Sonos devices. Wait another couple of minutes, then check in the controller for any Sonos updates, and then test.
If that doesn’t help things, I’d either call BT directly, or submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it. Don’t post the resulting diagnostic number here, they get sensitive about GDPR.
There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.
When you speak directly to the Support staff, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.
