I believe implementing MMS is outside of ModemManager’s scope. However, if anyone would like to help with MMS support before we eventually get to it ourselves (which may take some time since it’s not the highest priority thing on our TODO list yet), I believe the best way to start would be to read and get familiar with mmsd source code: https://git.kernel.org/pub/scm/network/ofono/mmsd.git/ and then try to write a ModemManager-based plugin for it (from a quick glance it seems its oFono-specific code is pretty nicely separated, so it shouldn’t be hard). I think this should be a task that a reasonably motivated community member might be able to easily take and progress on, which should speed things up and put the UI support higher on our TODO lists 
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