Why do people call from a restricted number?

I've been involved with a guy for a few weeks now. I was working on a project today that he wanted to be able to tell me how to do it and to keep instructing me through it (we live kind of far apart). So we spoke several times on the phone. He called once when I was at my neighbors house so I sent the phone to voice mail. I tried calling him back and got the beep that he was on the other line. Next thing I know he called me from a blocked number (which normally it isn't). What the heck...why do you guys do that?
Why do people call from a restricted number?
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