Why should I take your call?

Recently, I stoppped taking calls if the caller ID shows "Unknown" or "Private party". Now, really, why would anyone soliciting your services (I'm talking about recruters, primarily) would want to conceal their phone number? Especially when they don't mind leaving a message with a phone number! You really want me to try to write your phone number on a scrap of paper and then dial it instead of just calling back using the phone's records? No, thank you, I'm not even returning that call.