Telkom still billing me after porting my home telephone number

I have ported my home telephone number away from Telkom on 4 May. Their latest bill still charges a full month's rental for the month ahead, and when I contacted their contact center this morning, THREE times it happened that after explaining my issue, there was just silence on the other end of the line, so I cannot sort out the issue via their agents. I support request I logged on their web site has also been unanswered to date.