I apologize, but I believe what you are trying to cancel is the subscription you have with Sling TV.
Which would be a complete different product than what we offer here.
We sell Slingboxes which is a box that you connect to your cable box so you can have access to all the channels that you are already subscribed to with your cable provider anywhere around the world, and we do not have a monthly fee.
We are not affiliated with Sling TV whatsoever, even though we share the same name.
With Sling TV you don't need to have a cable provider, however you need to pay a monthly fee with them for the channels that you are subscribed to and have internet access.
This is the phone number to Sling TV : 1-888-348-1248
This will be Sling TV's website : https://www.sling.com/
Hope this helps!