What to do to cancel line?

Mailed cancellation of service form to servcancellation address on 2016/03/03, no reply. Went to branch in Mall @ Reds on 2016/04/30, submitted another cancellation form. Someone called me today from callcenter because only a dispute was logged from store (Dispute Ref ***0) - says I must call 10210 to cancel line... Explained that I was already in store to cancel. Was told she can't help me any further. What is a person to do to move/ cancel a line? Calling, mailing and going into store doesn't work.