The Cisco Telepresence Codec C90 is unable to call the Cisco IP Video Phone. Assuming that the calling search space has been configured correctly, which of the these can cause this issue?

The C90 can register to Cisco Unified Communications Manager, not only a dedicated Telepresence Video infrastructure server.

A multipoint control unit is only required for multipoint conferences, not for a simple point-to-point call between two endpoints.

The C90 supports both SIP and H.323 registration to Cisco Unified Communications Manager, so SIP-only registration is not a constraint that would cause this issue.

URI dialing is only required when dialing by URI address format; standard directory number dialing would still work without it.

The Cisco TelePresence Codec C90 has a system configuration setting at Profile 1 > CallSetupMode that must be set correctly (typically 'Gatekeeper' or 'Direct') to determine how the endpoint sets up calls. If this is misconfigured, the codec will fail to establish calls even when the calling search space and registration are correct. This setting controls the signaling path the codec uses when initiating a call.

If the C90 were not registered as a SIP endpoint, it would not be able to place any calls at all, but the question already states the calling search space is correct, implying registration is functional.