Hi there! Certain phones offer the option to block a contact, so you would save the number, then open the contacts and check the menus to see if the option is there. It may also show as "Direct to voicemail" in some models. You can also try downloading a call blocking app if the phone does not include the function.
On my phone, I hold my finger on the number in the call logs and a small menu appears. One option is "Reject Call" which adds it to a rejected calls list. Also Settings > Call > Call Reject lets me view or add a number and I can match exactly, match last digits, or match first digits ( 7187 seems to be a fav ending of 1 spammer ). Unfortunately, it doesn't stop them from leaving a VM, which seems to start automaticly if there is a sound ( stupid recording calls ), I hate that - if it's blocked it should just pick up and hang up or have a check-box to allow a VM ( to record someone making threatening calls ). Also, unfortunately, there seems to be no way to remove it unless that number is still in the call logs to bring up that small menu ( not in Contacts or Call Reject, I have just changed the area code to 999 in Call Reject ) but it will log a new rejected call. It should be illegal for phone carriers to allow someone to use a fake number but they love raking in that extra dough. If congress allows spammers to go direct to VM without ringing your phone, they should all be replaced. ( should be anyway )