Business Line not work for 3 months

This terrible service struggling now for 3 months with them with my business line that's not working. Been at them countless times with no results or answers. Losing clients because they cant get hold of us most clients know our land line nr for many years. Have no idea how it can be so difficult to get it to work again. All document they wanted they have but still it shows a different nr then our nr that we have preserved before hand.