I took this image with a phone (some Sony). The lens is clean on the outside. The owner of the phone says those black dots started appearing and supposedly are multiplying. Is it some dust in the insides of that camera or might it be the sensor somehow getting bad?
It's highly unlikely that this many pixels on the sensor would become "dead" in quick succession, or in this pattern (groups of pixels, rather than individual pixels).
This is clearly dust directly on the sensor.
I also see indicators of dust on the inside of the lens (larger, out-of-focus areas that are lighter than their surrounding pixels). For example, on the wall to the left of the top storage bin.
Looks really bad. Normally, dust gets on top of the stack of optical filter which sits above the sensor. It then shows as small blurry disks which get smaller and darker at narrow apertures because they are close to the sensor but not directly on.
From your example, I would guess that you have dust that entered below the filters and is directly on the sensor. There is no way to clean this yourself and you should therefore have it serviced.