5G icon disappears when wifi is turned on

I thank you for the comments.  I understand that I will not be using both wifi and the TMO network at the same time.  I am still unclear about what happens when i make a phone call without turning wifi off on my phone.  With wifi on, the 5g icon disappears.  Does the call still attempt to connect to the 5g network, or does it default to 4g.  I think most people, while at home, stay connected to their home wifi, even when not using it.  Having to disconnect wifi to make a 5g call doesn't seem right.