I amimplementing MS Exchange Server 2013 on a window server 2012 std O/S and currently have issues with Client Access Licence (CAL). 
I purchased 3 boxes of MS Exchange server 2013 (2x Standard, and 1x Enterprise) with each having 10 CALs; after installation and configuration of the standard edition, the product licence key for the disk was enter and the product was successfully activated.
But, I have couldnt find the means to add other product keys in order to use the CALs in the other boxes for this installation. Mean while we purchased extra 5CALs from a partner.
How can I use the CAL in the other two boxes and the open licence we purchased from Technology distribution.

You will need to ask your licensing rep these questions, no one here is qualified to answer licensing questions.

Hi,

Exchange CALs are honor system. It is not "installed" or "tracked" from within Exchange.
Please run Get-Exchaneserver to check the version number or editon, we can change it by run Set-ExchangeServer ExServer01 -ProductKey aaaaa-aaaaa-aaaaa-aaaaa-aaaaa. More details about it, please refer to: https://technet.microsoft.com/en-us/library/bb124582%28v=exchg.150%29.aspx

Here's an similar thread about your question:
https://social.technet.microsoft.com/Forums/office/en-US/c9dc2a4c-213c-4baf-9972-53b07263e896/how-to-install-exchange-2007-cals?forum=exchangesvrgenerallegacy

Thanks