You want to talk to someone, but not have them have your number and you go with the privacy excuse?
That's funny.
There are instances where this is more than valid so do not just make it out to be childish and or a privacy excuse just because it does not suit your own personal preferences/believes.
Many people have my number for business purposes and they are billed when they contact me for business purposes. When they however see me on WhatsApp they try and chat with me and I guess one can say 'score a freebie' and these chats are usually outside office hours, some even after midnight and if I do not reply I'm being told I'm rude etc..
I do however use chat apps to stay connected to family and friends. I thus prefer WeChat where I do not need to show my number and where I can use a user name which I can give to family and friends and thus not be bothered by people who possibly have my phone number.
Now is that childish?