I believe I understand your question since H+ is not 4G or LTE. You believe that when it does not say H+ the call is VoLTE, however when you place a call it switches back to normal calling and displays H+?
This link refers to the difference...
https://www.quora.com/Is-there-a-big...ween-H+-and-4G
From what I have been able to Google about this it sounds like VoLTE on your device will depend on two factors,
1) If your network support VoLTE.
2) Which SIM card you are using. Since yours is a dual SIM device this may be your issue. By all accounts I can find there is a way to set a default SIM and setting the correct one may be the answer to your question.
Your post is somewhat difficult to follow because English does not appear to be your first language. You can post in your own language and we can translate both ways to make it easier to communicate.