how can I forward all calls except one?
For starters, call forwarding happens at your service provider, not at your phone. For most services, to forward calls that you don’t answer, first dial *71+the 10-digit number to forward calls to. To turn off call forwarding dial *73.

Now, step two. Do a search on the play store for “white list”. There are a number of apps that will only allow the numbers that you have entered into the white list. In theory, all others will get forwarded. I haven’t tested it but it should work. Good Luck.