I currently have an Android Xiaomi Redmi Note 8 Pro which will no longer work when 3G is switched off next year. I had planned on waiting until closer to switch off to replace it, however I will be travelling to Singapore and Taiwan later this year and see 3G has already been switched off in these countries for roaming. 
I am currently with One prepay but want the new mobile to be compatible on all networks for VoLTE and Wifi Calling. The only way I see to guarantee compatibility is to select an Android make & model that the mobile companies all list on their websites, which mainly seems to be Samsung and Oppo.
I would prefer something in the Redmi Note 13 range as it seems you get better bang for your buck, and I was very happy with my Note 8 Pro, but am unsure how to confirm VoLTE/wifi calling compatibility. I am also trying to decide whether I should be aiming for a 5G capable phone as I tend to keep a phone for a few years.   
If I end up going with Samsung I was tending toward the A15 5G or A25 5G, or with Xiaomi the Note 13 5G. If I drop the 5G then the cost would be a bit less.
Any advice appreciated!