IIRC you had to do server side push as Windows Phone 7 didn't support local notifications and always-on Internet connection. Some IM clients used some tricks to

IIRC you had to do server side push as Windows Phone 7 didn't support local notifications and always-on Internet connection. Some IM clients used some tricks to run in background, such as masquerading as a streaming audio player (that had always-on capabilities enabled) but you lost the music player capabilities of your smartphone when running those apps. WP8 relaxed some of those restrictions but it wasn't enough to truly develop a IM client. It's true that only Microsoft could create such integrations, but it was a business decision. On Windows Phone 7 era, regular developers couldn't deploy native code and you couldn't call native APIs directly from the managed .NET/Silverlight runtime. Native SDK wasn't available at all, but it was a regular Windows CE at its core. Maemo's was way superior to Windows Phone. It's a shame that Microsoft trojan-horsed Nokia.