Focal length does not change with sensor sizes – what actually changes is the field of view:
FOV (°) = 2 * arctan ( d (mm) / (2 * f (mm)) )
FOV is our field of view in degrees,
d is one of the dimensions of the sensor (for diagonal, it is
d = √(h² + w²) ) and
f is the focal length in millimeters.
In very simple terms, a smaller sensors sees a smaller portion of the lens’s projection – e.g. a APS-C sensor does not fill out a 35mm lens’s image circle, so it cannot see the outer portions (which relate to the outer regions of the frame) and therefore, the field of view is narrower than it would be with a full frame camera.
When we talk about crop-factors, what we really are talking about is the change in field of view by a smaller sensor: The reference is a full frame sensor, which is 36*24mm. Therefore, a 10mm lens on an APS-C camera is still a 10mm lens – it just happens that it offers the same FOV as a 15mm lens would on a full frame camera.
The same of course is true for smartphones – unfortunately, the smartphone market is constantly and rapidly changing, and manufacturers usually do not state focal length or sensor size in their marketing texts, so research is needed on a per-phone-basis (or so I think).
A (at this moment) recent iPhone X offers an equivalent-to-28mm lens and an equivalent-to-52mm lens, the Samsung Galaxy S9+ offers 26/52mm, the OnePlus 6 offers 25mm, the Huawei P20 offers 27mm/80mm, the Nokia 6.1 offers 27mm,…
So it seems we can conclude that 26-28mm (equivalent) are a standard in Q3/2018.