The default ringtone on my iPhone is set to "sonar". As I build my contact list each number is assigned a different ringtone depending on who they are - family, friends, business contact, etc. People who don't leave a voicemail don't get into my contacts.
When I hear "sonar" I usually ignore the call but I may decide to answer out of curiosity, or I may answer if I'm expecting a call. I might also call back later to identify them - my phone, my option.
Now I'm starting to get junk text messages - not sure how to handle that traffic yet.
Hope this helps.