There's no need to split it into several parts like Modem Daemon and KDE Telephony Daemon, since the plugin is not KDE-specific and could use any implementation that follows the appropriate XML D-Bus API description.
Work-in-Progress.
Getting started
Minimal iteration could contain these packages:
kde-telephony-meta (D-Bus API descriptions)
modem-daemon (calls and history) — provides d-bus adaptor according to kde-telephony-meta
kde-telephony-plugin-declarative (QML plugin) — provides d-bus interface to kde-telephony-meta
After building these packages and running /usr/lib/libexec/modem-daemon via /etc/xdg/autostart/org.kde.modem.daemon.desktop you should be able to play with USSD via QML interfaces like this:
The USSD is probably the simplest part of the API to start with. The same logic could also be found in the plasma-dialer project. Feel free to provide feedback via the issues list of the appropriate repositories.
Future Work
In theory, it could be added to the Plasma Phone Components repo, since the current ModeManagerQt QML plugin is there right now.
After that, the next things could be ported to these plugins:
The part of the Plasma Phone Components called mmplugin could be ported to Modem Daemon (after it will be ready; when SignalIndicator API will be ported here).
The part of the SpaceBar (SMS/MMS Application for Plasma Mobile):
Not related to KDE (MMQt-only) could be ported to Modem Daemon
Related to KDE (contacts and avatars) could be ported to KDE Telephony Daemon