That really sucks! But i wouldn't panic just yet, as we sometimes see that Telkom crosses lines at the exchange and this appears as a different number, until they rectify the mistake. An easy way to tell would be to switch of your router, or check if your logs show more than one number connecting (the other being your own number). If there is only one number connected. If there is only single number connected at the time you are online, chances are it's acrossed line and must be reported to Telkom.
If not, you will need to secure your DSL password and possibly change it to a more complex one (as it may have been compromised). You can do this in ClientZone at any time, and it will take effect as soon as you disconnect and start a new session (so you will need to change the password). Please also ensure that you use a secure password on your router. Most routers use standard IP address for convenience, so if someone is able to log into your router, they could compromise your password on some models which allow an unencrypted view of the password.
Don't ever give your ClientZone or DSL passwords out (even to us) to ensure that you are never in a position where someone may use your account without your permission.