I had this exact same problem on a Samsung Galaxy S10 Lite but solved it. I got the same answer from Tech support with them stating that it can't be changed from 20 seconds. What I did was put my Sim in an older phone (Samsung Galaxy S5 Neo) and sent the code **61*+447953222222*11*30# from that phone. I then checked to see if it had worked using the code *#61*11# and that confirmed the time before going to answer phone was now 30 seconds. I then put the Sim back in my S10 Lite, checked the time using *#61*11# and also by ringing it from another phone and timing it; definitely 30 seconds now. Just for interest, I also tried the Sim in a Samsung Galaxy S6 Edge+ and I could alter the delay time on that phone as well. If you don't have either of those models try it in another older phone; I'm fairly sure it will work in quite a few. I phoned Tech Support back and told them how I did it, so hopefully they'll suggest my solution to others with the issue rather than giving incorrect answers. Who knows, they may even investigate why it doesn't work in some later phones and solve the problem for good, but don't hold your breath.
