All it say is
This means your data could be slower than normal. We're sorry about that. Our team is aware and working to make it better. Our top tips for staying connected would be to find local Wifi hotspots and download whatever content you need whilst connected to Wifi, so you can still enjoy it on the move.
Which to be honest it’s been saying for the past few months, not much help when your home WiFi is down and you need to use your mobile. Fortunately my broadband connect has now been restored so I can again use WiFi calling but I think for me this has highlighted that the move to O2 is going to work for me.