On newer, dual sim iPhones the dialer keypad screen has a small pane at the top that has displayed the last resident number used for the line you are dialing. A tap will on it will open a popup “Select the line would like to use now.” And your options. Easy Peasy.

Also be aware that iPhone 12’s & newer will have 5G service, and on T-Mobile this is a valuable upgrade for coverage and data speeds.