same problem...here's what I do...I save each number they call from..when I add it to my contact list I put name as "No Answer"...than I go to "Ringtone" and mark it with "NO"..it will vibrate but, it's not as bad as if it rings. BTW....I owe NO ONE...they are trying to collect from a dept my mother had when she past away..8 years ago....no matter what I do THEY KEEP CALLING!! oh....and they have A LOT of numbers they call from..I have 4 slots on my phone to add their numbers to under each name I choose to call them on my contact list.
If it is a wrong number, just call back and let them know. No need to block the number because you will probably still get the calls from other agencys.
Infinity Group collecting a debt of mine from like 5 years ago, told them I don't pay collection agencies (probably the 3rd agency that has handled this debt). It's almost off my record!