Carriers pay to port numbers to a neutral third party that maintains the Number Portability service. In the US, that used to be $6 per number port (which is basically just a DB row update operation) but a new vendor has reduced it by more than half. Carriers view that expense as pocket change as subscriber acquisition cost is well over $100 in total.

Without a legislative mandate and a third party integration partner that assures the port is successful, seems unlikely that a market leader like Spotify would ever allow them to exist.