Truecaller and privately are antonyms.
Truecaller works by uploading all of your contacts into their database, it's a crowd sourced database.
Someone recently on here was asking about how to keep their number private. Apps like Truecaller are the reason you can't keep your number private. As soon as someone who has truecaller puts your number in their contacts list, it's over. Same idea with Facebook. It's an OSINT contact exploitation technique. If you have a number that you keep private and give to only a few people, but I want to identify the number owner, I simply take that number and search it in truecaller. If anyone who has your number in their contacts list has true caller, now I have the name behind the number. Same idea with facebook. Your number is my target, I can't find any accurate information on it after researching it. I make a facebook account, add your number to my contact list, give Facebook permission to my contact list, and now im getting friend options from "people I may know" or rather, who you know.
Remember now, even if you have the number registered as an alias, if you're john doe and im your friend and know your true identity and save your anonymous number as john doe in my contacts and the contact then gets uploaded to facebook or truecaller, its getting uploaded as john doe and not the alias you registered it as.
Truecaller is ruining all your spook friends' spookiness.
At least this is my understanding, someone correct me if im wrong.
Also, there are tons of free OSINT webistes you can use to run numbers for owner information. Most normies are going to have their true ID registered to their number, it's pretty reliable. Just ignore the number, look it up and call them back if it's someone you should be talking to.