If you're getting your subscription through your mobile provider, it's them who cancel the service, since it's them who charge you for it in the first place.
Your Spotify account gets linked to your mobile number and that's it. No payment info is recorded in Spotify's systems, and therefore no charges can be made by them.
If you receive or have received Spotify payment receipts, then you can cancel in your subscription page.
Hope it helps. :)