Welcome back to the community and thanks for taking the time to post.
I'm sorry to hear that you're having issues with WiFi calling /4g calling.
1. The Wifi Calling and 4G calling compatibility device list has not been updated nearly for a year. I would like to know whether VoLte / Wifi Calling is supported on my mobile?
I have looked into this and cannot see that the service is not compatible with your device at the moment.
2. Is Wifi Calling and 4G calling enabled to all customers by default in normal case?
The service would need to enabled by the customer.
3. The 2G/3G service will be suspended in the UK very soon (by the end of 2023). If 4G calling / Wifi calling is still unavailable on my device before the deadline, can I terminate the contract without any condition?
No you wouldn't be able to leave with out condition as 4g would be available on your device as we offer a network service, not a feature service.
Hope that answers your questions.