time consuming, but you can accomplish this.
1. go to menu.settings.sound.phone ringtone from here set up the ringer for the calls YOU DON"T want to rush to. You could also set this to silent..then only people in your contacts list will ring.
now go to people, and then go through each individual contact setting up a custom ringtone for each. it can be the same ringtone, or it can be different that is up to you. as long as it isn't the same as what you set in step one you will know the difference.
another tip...when you get these unsolicited calls, save the number to your contacts as the name "unknown" . do it each time and occasionally go in and merge/link the all of these "unknown" contacts. set them to ignore them by using the "send to voicemail" feature. your phone never rings..they just go straight to voicemail.
there is an app out there called privacy star something or another that has many many call features. if you are getting a ton of these types of calls you might check into that app