so this is how it look on my phone.
I called a hertz location, then i pressed add call and called a second location.
at this stage the screen has a option merge (see pic 1). after that there is a option manage (see pic 2).
when i press manage i can hang up on either one of the conecctions (see pic 3) or i can even add a 3rd cal l(see pic 4).
i have a custom rom installed now but the stock phone app on tmobile is exactly the same.