IPCC Enterprise is not supported in a coresident deployment like IPCC Express. In other words, you cannot have CCM and IPCC Enterprise components on the same server. 125 agents is the capacity if this server type is used as an IPCC server. 2500 phones is the capacity if the server is used as a Callmanager.
Theoretically, you could double the number with a Pub and a Sub. You could have 2500 phones registered to each server. The problem with this is that you would have no room for failover. Ideally, if the subscriber is down, those 2500 phones would register to the publisher, but in this case the publisher would already be at capacity.
Hope this helps. If so, please rate the post.
Brandon