Unfair billing

I got a wifi router form Telkom, from the guys who go door to door to sign us up. He said he will come and connect it as soon as I receive the router and when he was supposed to come he then said i needed to pay him R299, which I refused because it was not part of the initial deal. Well the router arrived and it was NEVER connected BUT Telkom now claims that I owe them close to R5000. I tried to call telkom numerous times, but I do not get assisted. How am I owing for a service that I never used? This is now affecting my credit score, and I need it resolved.