Whenever I visit new countries, I often find myself searching, looking, and asking around for the national phone number to close my flight plan. I think it would be incredibly beneficial for all of us if the ARO phone number for the country we're currently in could be easily accessible, perhaps in the radio tab or a similar section within the app.

No, you have always been able to close your flightplans via SkyDemon. Normally used when you've landed somewhere without facilities to do so.