Gotta ask: have "Silence Unknown" turned on on iPhone?
Have not seen spam calls on my iPad and have above turned on.
Also: what call blocker does you cell service provider use? Is there an app you can download and set it so that spam calls get blocked or sent to voicemail?
Again, got an app (ATT's Call Protect) that filters out the known spam callers, so with the two pieces in place, my phone is pretty much silent these days.