Limitations Office Communications Online (OCS) in BPOS
Office Communications Online (or OCS Online) is great but has some limitations compared to an On Premise OCS implementation.
First of all there is no coexist scenario (like Exchange Online). Therefor it is not possible to use both Office Communications Online and OCS On Premise and exchange IM's between the two.
Audio and Video is limited to computers which are on the same corporate network. There cannot be any firewalls or other network traffic control devices between them. Of course this is not the case for IM's (IM's are routed through the server, Video and Voice call are set up between to clients).
Federation with other networks (like Windows Live) is not active.
OCS Voice is not supported.
However if you only need OCS IM and presence info you will be able to use OCS Online. If you only use Office Communications Online from the BPOS suite, you can integrate with an On Premise Exchange Server. So in a scenario in which an On Premise Exchange Server is active you will be able to display presence updates and Out-of-Office information from the on-premises Exchange server. This is done through the On Premise external Exchange Web Service (EWS) URLs.