Ignoring the technical challenges and impossibilities of open sourcing it (Windows Phone 10 is Windows 10, and the Windows Phone 7/8 codebase was its own crazy beast that was tough enough to work with even when you were being paid to do so), mobile operators do not want you running custom code on their stack, and they are the ultimate gatekeepers of what devices are allowed to connect. There will never be a popular, easy to setup open source phone without a complete reworking of cellular service, at least in the US.
Apps already existed, it was around the time no one knew which platform will end up being most popular, so many apps had their Windows Mobile version. Not nearly as many as on Android or iOS, true, but they did exist. Especially in Europe.
I don't think Windows Phone 7+ included anything wild on the baseband. It ran on qualcomm chipsets, and I suspect it was just using their modems as-is.
Use an approved modem blob and you can connect. Easy peasey. VoLTE makes it a bit trickier, but not that much.
I mean, yeah, it won't ever be popular. If it's open source, there's no way to sell user data to subsidize the price. And who wants to pay full freight for a phone that probably won't ever fully work... I think I learned my lesson with OpenMoko...
I enjoyed Windows Phone, but I don't see how it would survive without big pockets and a dedicated, focused, and engaged team that can m move mountains. Big pockets alone weren't enough. Good hardware wasn't enough. Microsoft needed to have delivered on their promise of all WP8 devices getting an upgrade to WM10, and WM10 needed to be good on release. Incidentally, what I think was the final build for Lumia 640 was a lot better than the first build; if the first build was that good, the story may have ended differently. Mobile Edge was still garbage on that phone though; Mobile IE was ugly, but Mobile Edge put a queue between the UI and the engine, which meant when the engine was spinning on some page it could barely handle, you couldn't stop it... All your UI commands would be queued up and played later when the engine had finished whatever it was doing... Unless the watchdog timed out and the whole phone rebooted.
You can buy a module that plugs into a Raspberry Pi and interfaces with cell networks. Many years ago someone wrote a phone program for one of these modules and called it the PiPhone.
Ignoring the technical challenges and impossibilities of open sourcing it (Windows Phone 10 is Windows 10, and the Windows Phone 7/8 codebase was its own crazy beast that was tough enough to work with even when you were being paid to do so), mobile operators do not want you running custom code on their stack, and they are the ultimate gatekeepers of what devices are allowed to connect. There will never be a popular, easy to setup open source phone without a complete reworking of cellular service, at least in the US.