This is the new tactic by robodialers to circumvent blocked numbers/systems. By doing this, the second call might get through via Call Waiting (if you have that feature) and therefore does not get blocked. Has happened multiple times to us on our landline with that feature.
We don't get many spam calls to our cells, fortunately, but this "REWARD REDEMPTION" SCAM is the most frequent one. I STRONGLY suspect that our carrier, NET10 ~OR~ the source of our pre-paid minutes - CALLINGMART.COM - is SELLING our number to the scammers for this, especially since we always pay via PayPal, so they never get our credit card numbers. They WANT them! In fact, I mostly suspect the latter - CALLINGMART. We get one or more of these calls within the first few weeks of adding more minutes to our phones. Also been getting spam texts trying to get us to go to websites. We ignore all of them, of course, but can't block on our cheapy phones so continue to be vulnerable to this crap.