cancellation, now debt collection

If there is anyone that actually cares about doing their job on this forum I need some assistance. I canceled my policy months ago. When I canceled, I paid off my last billed and all devices. When I went into the store to cancel, the guy assisting me didn't cancel my digits line as requested. I haven't used the line or the device attached to that line in over a year. I was told by the representative on the phone they would get the bill erased since I had the lines canceled and hadn't used the devices even before I canceled the lines. Now they wanna send the bill to collections.