Yep, you set the Security String "DC" for all DataCenter objects, you create then a "VOIP" Security String for all objects in the VoIP area, and then you set up two populations of users, you remove the "ADMIN" Security Community for both populations and only set the VOIP one to the VoIP Users and the DC one for the DataCenter Users. They would then only have access to their objects.
I also highly recommend you don't mess with Security Strings in the Universe containers but rely more on Global Collections. You create two Global Collections (one for VoIP objects and another one for DataCenter objects) and you set the Security Strings accordingly on both of them. After that, do the same things as explained above on your users (giving them the appropriate Security Communities) and you will have the expected outcome!