UC540 has more DSP resources and larger Flash memory, and priced better than UC520, and will handle up to 32 phones per site, and up to 5 sites in a full mesh (IPSEC) data sharing environment, with extension dialing among all sites, all configured with CCA.
UC560 goes up to 104 phones if you need more per site.
But 5 sites is the limit.
Look at slide 5 as to how features work across sites:
https://supportforums.cisco.com/docs/DOC-9701
Did I answer the question?