Which will the phone choose to use during calls if VoLTE and Wi-Fi calling are both turned on?

When your phone is using mobile data, VoLTE is the first choice. When the phone is connected to the Wi-Fi, it will use Wi-Fi calling first during phone calls.
Wi-Fi calling is a feature that allows you to make and receive phone calls over a Wi-Fi network instead of using your cellular network. This can be especially useful in areas with poor cellular reception but strong Wi-Fi signals.
VoLTE stands for Voice over LTE, which is a technology standard for making voice calls over 4G LTE (Long Term Evolution) networks. Traditionally, voice calls have been handled over 2G and 3G networks using technologies like GSM and CDMA.