voicecall is a daemon (and QML plugin) for implementing dialer UIs.
voicecall-manager is a system daemon responsible for collating and managing the telephony subsystem for the dialer UI to interact with.
the dialer instructs voicecall-manager to make calls, which then subsequently uses plugins (such as an ofono backend, for instance) to make the actual call.
likewise, the daemon listens to various backends in order to communicate incoming calls to the user interface.
communication between the user interface and the manager occurs over dbus, on the org.nemomobile.voicecall session bus interface.