Some people wanted access to release builds signed by first party keys, or release builds that come out the same moment as the release does, so this is how those are being distributed. For most people we would still suggest to prefer F-Droid.

Sorry for the slow reply, we don't always check our replies often. Hopefully you got this resolved but if not feel free to reach out to JMP support jmp.chat/faq#support

Yes it works anywhere you have internet and an XMPP-powered app

To be clear, you can do more calling, but after the 120 minutes you pat a little per minute jmp.chat/pricing for more

It should work in any device that takes a SIM card. Profiles can only be added using an android device or a USB reader, but can be switched using any device supporting standard SIM Toolkit menus.

I believe it should work but haven't tested extensively that use case.