Daniel,
A user can be configured for either Remote Call Control or Enterprise Voice, but not both obviously. It sounds like you have your environment configured for using RCC only. Selecting Enterprise Voice would only use OCS for voice calls and not integrate with the hardware phone at all; calls would be routed over OCS to the IP-PBX only skipping the handsets. In RCC mode only Communcator-to-Communcator calls would work on the computer itself, while all Inbound/Outbound PSTN calls would terminate at the physical phone.
So basically you can have handset integrate OR VoIP using OCS. I don't believe there is any way to have RCC enabled but then use OCS to receive voice calls to a physical phone extension over the computer's audio hardware.