I will usually text someone before I call. It’s just polite, and my texts will usually come through even though they might have some sort of spam blocking service.
Silence unknown callers doesn’t just check your contacts. It looks through your messages, out going calls log, and emails. If it finds the number that you’ve previously had any sort of dealing with, it won’t block the number. If I called someone who has it set and get sent to voice mail, and they call me back, their call won’t be blocked because I’ve previously called them.
Setting Silence unknown callers has been the only successful measure I’ve undertaken that has ended the endless interrupting stream of spam calls.
I guess if you have a customer service type of work and expect dozens of calls from unknown numbers, it doesn’t make sense to use Silence unknown callers. However, a friend of mine who has an appliance repair service has it set. 90% of his customers are repeat customers and if one does slip through the net, it just goes to voice mail anyway.
I used RoboKiller and it sent my head hunter’s call to oblivion. He got a message my phone was disconnected. Not cool. With Silence unknown calls set, the worst is he’d be sent to voicemail.