How to set a ringtone for unknown numbers on my iPhone?

I have a 3GS and three ringtones for separate groups of contacts. What I need to know is how to set a ringtone for any number NOT in my contacts list? I'm getting so many junk calls its doing my head in. And yes I'm registered with TPS and O2 Spam Alerts, if anyone wanted to suggest that, but it's not stopping them!

You can't. You can set ringtones for groups and probably for individual people (you can do this on Android). Otherwise, the phone will use the default ringtone. The phone has no way of knowing if a call is from an "unknown" caller. It reacts strictly by the incoming number.

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