With CM 4.X and prior the licensing was not enforced and the license was only good for putting it on a shelf in case auditors come by.

With CM 5.X and up the licenses are actually enforced via Device Unit Licensing, where each device takes up number of DLUs. For example 794X and 796X take up 4 DLUs, IPC takes up 3 DLUs, etc.

So before you add a phone to CM you have to make sure you have sufficient number of licenses available, otherwise you will not be able to add the phone.

HTH,

Chris