luceos A raw IMAP option might be possible but would either be commissioned or contributed.
How many working hours it may take?
I think users can start a discussion using WhatsApp, we can relay on API providers like twillo or kaleyra
A user sends a "hi" message to a WhatsApp number > selects options > starts a discussion... this will be an expensive API call for every API request WhatsApp charges around 0.013x4
the possibilities are endless, we can also integrate this extension with IFTTT
using the android app of IFTTT we can create something like if someone sends us an SMS we can start a discussion
if someone tweets something we can start a discussion