Most likely explanation: if you do not use Skydeck’s voicemail system, calls that go to your regular voicemail system may show up in your Skydeck account as calls to your own phone number.
Possible: a call from a number that your cell phone company didn’t recognize may show up in their records as a call to your own phone number.
Rare: if you move to a new metropolitan area and then port your phone number from another cell phone company to AT&T, AND you have a certain kind of phone, ALL of your incoming calls will appear to come from your own phone number. This is a problem with AT&T’s billing system.