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I am below the confirmed openreach minimum download speed which is 44.38Mbps. I have never gotten the guaranteed speed and the estimated download speed range. I have the Superfast 2 package and I had 49Mbps now I have 44Mbps. It has been months since I claimed the discount and it still haven't been fixed.
The discount is triggered by using the mobile app. Once triggered you should continue to get it until your "sync speed" is greater than the global broadband guaranteed minimum of 55Mbps (you don't need to continue checking it with the app). If you are below your personal minimum guaranteed CONNECTION SPEED (ie not the sync speed), then you should CALL TECH SUPPORT to arrange an engineer to see if there is a fault. If you are waiting for Vodafone tech support to call you, it'll NEVER happen!