Like I have said earlier, the icon will only disappear after you have asked Telco to deactivate the service. You cannot disable VoLTE via phone settings. The icon will still be there if you only "disable" from phone settings.
VoLTE uses your 4G data. But it will not use up much data. Very very small data use as your call will not use a lot of data. You will use more data when surfing the web than when calling.
Note that VoLTE will only work if the person you are calling is also using VoLTE. Otherwise, the call will drop to 3G. For example, you are calling someone's mobile number and that person has VoLTE, then the call will be using 4G. If you are calling someone's office number and the phone is not a VoLTE-enabled phone, then your call will not use 4G but will switch to use 3G instead.
VoLTE, because it uses 4G, will have faster connection. You will dial through faster compared to non-VoLTE calls. The waiting time to hear the dial tone will be faster (almost instantaneous) and you will have better clarity over your calls.
I am not an expert at the call technology, just my experience from using VoLTE. Apology if there are any wrong information being presented.