Why hasn't my boyfriend of 2 years given me his second mobile number,which I am aware tha he uses?

So my boyfriend has never given his second mobile number (I am aware that he has it and has used it around me, so its not a secret or anything), anyways that's where he actually uses whats-app on. I have told him to add me since I only have his other mobile which is the non smartphone (Burner how some people call it) where he has family members me and I don't know who else, but every time I mention it he seems reluctant or the topic changes right away (Poof!!) and I stay with the same doubt over and over and over again.

The point is, that 1 year and a half have passed by and I still don't have his second mobile number (which happens to be a smartphone, cause the mobile number I have is from a small non smart phone aka' the burner how people call it'') for us to communicate on whats-app, Which I believe is pretty normal for boyfriend and girlfriend now a days right? Ok

What in the world can this 37 year old men be hiding from me, if he has not given me his second mobile number in which he actively uses Whatsapp on, I don't get it, and even if he had something to hide and does not want me to know , how in the world would I figure it out through his other mobile number and Whatsapp, its impossible so what is he so afraid of, I don't understand Really.