When I switched from a sprint phone to a T-Mobile Samsung A32, I had the same problem. My other phone, a Sprint Samsung 10, with a new T-Mobile SIM card still was able to change Call Waiting thru thru the phone app, under supplementary services. But the A32 was not able to do that, nor able to do *72, which my old phone was able to do for years.
I spent 5 hours trying to get the A32 to work for CW, but no way. I found out yesterday, that it might be the phone. S10 should be able to do CW, but I guess A32 does not. It would be nice to know this before I bought the phone, as that was the main use for my second phone, to sit in a drawer and forward calls.
There does not seem to be a list of Samsung phones that allow CW, nor does anyone know. So rediculous, since CW was been around for 40 years.