Is it possible

So me and my husband are very open with each other. Well I noticed a few weeks ago on his phone he had a phone call that came in and he missed it well then he called it back. The only thing is, is it was a private number. I didn't think that was possible. Well now I seen it again where yesterday there was an out going call for 5 minutes to a private number and then today for 3 minutes to a private call. Can someone please explain to me how to even do this. Cause I'm really trying to rack my brain for this one.