Did you add the number manually to the reject list, from the contacts, or from recent calls?
I ask because I've had a similar problem before back on my S4. Adding it manually or from the contacts didn't seem to work. Then in the call log, I long pressed the recent unwanted call and added it from there. Only difference was recent call number had three +1 in front of number. After that, I had no more problems with it.
Hope this helps yea.