It is just that Microsoft as a whole viewed the Internet at the time as a "not-invented-here" competition that they could overtake with proprietary breakage like Internet Explorer...
What about the many SIP-clients available on the AppStore?
I use XS4ALL SIP-client every day, and it reroutes my VOIP too!
The world is bigger without Nokia!
What about them? They require a separate app, Symbian had it built in, and the "Internet calling" capability was integrated directly with the contacts, you could choose to call using cell phone or SIP from the same dialer (including using 3G data, not only WiFi). It is just one example of things that have been standard on Symbian years before other platforms had them. Another example, the panorama picture feature on iPhone 5 - that was available years earlier on Symbian. As I said, it is not one single thing, it is the combination of OS features and hardware capabilities that was superior.
What is worse is that it is hard to find any existing Symbian devices...
For all the drawbacks of Symbian, the combination of a camera that put to shame any other cellphones, and the built-in capabilities of the phone (e.g. a complete SIP stack, integrated with the regular phone functionality) is still unmatched. Even Nokia themselves cannot replicate the hardware capabilities of the 808 in a Windows phone, because the OS can't handle them...
One of the chief duties of the mathematician in acting as an advisor... is to discourage... from expecting too much from mathematics. -- N. Wiener