How can I ask my boyfriend for his other mobile number, If he wont give it to me?

My boyfriend has another mobile number and I want to have it as well as the usual one that he uses, but I have asked him a couple of times and he just won’t give it out and I got tired of asking so I let it go, the problem is that he uses that number with a smart phone and the number I have is from a plain small phone. I find it weird since nowadays most people interact with smart phones. Why would he not want me to have his smart phone number, can he possibly be hiding something from me.