Impossible to cancel internet subscription

We are leaving Belgium by the end of this month and need to cancel our subscription (internet + TV).

The problem is that the only way to cancel a subscription is a line (04 266 41 14) that is operated by people who don't speak English. So I called 4 times already, they always tell me that someone who speaks English will call me the following day. But nobody ever called me back. And for the record - the operators understood pretty well (from my google translated sentences) that I need to cancel my subscription by the end of the month. 

I also went to one of your branches - Voo Stockel. The lady told me she cannot cancel my subscription - only the line 04 266 41 14 can do that.

So this is a vicious circle and everyone who doesn't speak French should be aware of this.

What am I supposed to do to cancel the subscription?