It seems to me that this area intersects with the PIM one. Are we sure we want applications organized this way? It may sound more difficult to implement but I think the most logical design would be the following:

• A contacts app manage contacts and provides API to other (third-party) apps to read and update contacts list. The UI let you browse contacts and manually update them. A call button on each contact launches the dialer app.
• A chat application (Konversation maybe?) cares about SMS in PM 1.0 and in future releases support open protocols like IRC, Matrix, XMPP and Telegram as plugins/backends (like Telepathy did). It reads and updates contacts.
• A dialer application that interact with contacts too

Third party apps using the contacts API may include e-mail client and calendar apps. I also think that this should be part of a Freedesktop standard defining how an app can provide PIM informations and how others can get them, so that other projects could adopt it and maybe make it working between Flatpak applications.

That sounds like a good plan for the future, but currently, the functionality is split. Koversation provides IRC and Matrix capabilities, Kaidan is used for XMPP, Telegram is supported by the official app, and there is a basic app for telephony.

Developing a rule-them-all messaging framework doesn't sound like a goal for PM 1.0 (i.e. something to focus on)...

Since we have basic functionality already in place, I am closing this meta-task. If there's any further improvements, we can create separate smaller tasks.