I’ve been looking at the forum about this problem, but I only found a couple of entries from two years ago and referring to the same alternative: NoPhoneSpam, which I tried with no success.
The thing is I found an entry about the topic where it is said that this feature is included in Android 6. My question would be as follows: would this feature be included in FP Open? I couldn’t find any reference to this.
I think you need to add the contact beforehand and then block it through its contact card (“redirect to something” was the option, IIRC). I’ve not personally tested it, though, since I’m in Lineage OS.
I guess there wouldn’t be any difference in that case . Actually it might be a solution, I would just have to disable my voice-mail and create a “Spammers” contact so I’ll give it a try before installing LineageOS