Cisco's formal HCS offering, supplied by approved partners*, is the only official one these days. There used to be partners offering other options where multiple customers would co-exist on one CUCM cluster but the CSS/partitions would keep them separated. To my knowledge, these are all deprecated since the HCS announcement. Of course, there are non-Cisco offerings out there too.
The HCS offering is actually pretty decent, mostly because it's the same CUCM/CUC/IM&P/etc. products that customers can install on-premise. There are essentially two differences:
- The required infrastructure to setup an HCS pod is prohibitively expensive for anyone except the big players. It's mostly carrier-grade equipment that an end customer would never buy.
- The products are abstracted behind a provisioning/billing application (telecom parlance calls this Operations Support System or OSS). Last I checked this was Voss product that Cisco was OEMing. This is the only difference that really matters to you because it sets up the dial plan and such with some assumptions that you wouldn't otherwise be confined to if programming CUCM directly.
Editorial comment: Whether this is good or bad depends on the customer. CUCM is very flexible because it has such a huge install base across so many market sizes/verticals. It can be shaped to fit most custom requirements; however, the OSS toolset takes a far more cookie-cutter approach.
Other than the OSS toolset, it's essentially a normal Cisco UC deployment: they simply connect the VRF instance for you as a tenant in HCS into your WAN. After that it's effectively a Centralized Call Processing model. Normal voice gateway/SRST options at the offices and IP phones or Jabber for users.
*Full disclosure: my employer is one of those partners.
