Why am I unable to do a conference a call over bluetooth?

[AC Question]

cannot conference a call over bluetooth

i have a Samsung j and i tried to connect it to my car blue tooth, it works fine until i had to conference a call or answer another call while i'm on a call and it wont work, i had to take it off blue tooth and put it on speaker so i can answer the other party's call, is this a common problem with Samsung? coz I had the same issue on my old phone ( Samsung s4)