Microsoft has applied to trademark what appears to be a ring tone at the USPTO.
Given that the only phone Microsoft is producing is the Surface Duo, the natural assumption is that this is the default tones which we will be hearing when that device rings. The sounds can be heard in Microsoft’s Surface Duo introduction here.
The trademark, however, covers a wide range of devices, from PCs and laptops to smartphones, so the sounds may have a wider application than that, with Microsoft noting it can apply to:
Computer hardware; computers; laptop computers; tablet computers; notebook computers; mobile computers; mobile phones; smartphones; handheld computers; touch screens; computer hardware peripherals, namely, headphones, computer keyboards, computer mice, computer stylus, headsets, electronic docking stations; software; software for mobile phones; software for computers; software for tablet computers
Have a listen below:
Either way, the dulcet tones can be viewed (and even downloaded) at the USPTO here.