This button / feature should only be used when you do not want to continue calling the rest of the numbers on this account. For example, spoke to the right party or got a payment / promise to pay.
When the call ends, you will see the ‘finish call’ button turn into an icon that is a circle with a line through it. It will only be there for about 5 seconds, so you should click that as soon as the call is finished. It then will pop up a “warning,” you will then click ‘OK’ to remove those numbers.