Sky mobile Wi-Fi is on but Sky state some phones provided by other networks may not be compatible, this is the case here. EE seem to have made setup adjustments that prevent the S9 from working fully with other providers. Whether a factory reset would put it back to Samsung settings or not I don't know. That's an awful lot of work if it didn't work (even with backups & restore) and wouldn't go down well!
Yeah guess the various carriers do put some of their own stuff on !
Since you know is turned on by Sky could try :
I did have a problem on my Note 10+ with WiFi Calling when updated to Android 11 UI 3 - don't know if will work on your S9 ( or If the settings will be in the same place ) Go in to Settings - Connections - Mobile Network first check mode is set to Lte/ 3g / 2g ( I believe you need 4g ( Lte ) and to be connected to WiFi) Then go to Network Operators ( is on same settings page ) turn off Automatic and will then ask to scan for available networks - tap Sky on the list. Then check if WiFi Calling back This worked for me on Vodafone but not for someone else Only other suggestion I could make is turn phone off- take out Sim - re insert Sim and Restart phone Hope one of these work
Just another thought - if these don't work might be worth clearing the phone's cache ( don't lose any personal info on the phone ) worth checking on Google or somewhere how to do it but THINK is power off phone - hold vol up and power button at same time till phone boots up - should get a menu at top - use vol up / vol down buttons to scroll up or down - highlight wipe cache and use power button to select - wait a moment or two to give it time to clear it then restart phone. ( As I say would just check on Google or somewhere for the procedure in case I'm not quite right )