Windows Server licensing question

We have a Windows Server 2012 standard R2 and a Small Business Server 2003, both servers are on the same subnet. The 2012 server is a domain controller (domain A) and all users on the network have joined that domain and log into the 2012 server. The SBS 2003 is also a domain controller using a different domain (domain B), none of the users are joined to domain ‘B’. A handful of users that belong to domain ‘A’ also access the SBS 2003 (domain B) using a web based application.

25 users in total are joined to Domain ‘A’ and log into the 2012 Server – of those 25 users only 5 users access the SBS 2003 (domain B).  No email is hosted on either server. User cals are being used.

My question is what are the licensing requirements for each server?

Right now we have (25) server 2012 user cals and (5) SBS 2003 user cals – is this correct? Or does the SBS also require 25 user cals?

Thanks