it depends wheter its what phone you are using if its an Android based phone for example you can find that number behind the battery usually or with simple commands on the phone itself,
e.g. (Android) https://www.verizonwireless.com/b2c/findDeviceId/helpOverlay.jsp?pageName=androidPhone
(Iphone) https://support.apple.com/en-il/HT204073
that number just tells unity that any ads running on that device are test ads and treat them so.