Bill collectors have been known to spoof caller ID with relatives' phone numbers. Yes, I know this is illegal, but it is still done. The best approach is audio

Bill collectors have been known to spoof caller ID with relatives' phone numbers. Yes, I know this is illegal, but it is still done. The best approach is audio captcha where you have to enter a random 3 digit code to get the phone to ring. This throws off most call centers because, the part which makes the phone ring is automated. Once the line is answered, an agent is connected to the line. If the collection agency does not use automated dialing, then DTMF passwords may be necessary. This is a pain for friends and family, but it can be a very effective filter.