Bogus debt collectors company

I have been receiving multiple sms from Hugo & Ngwenya Attorneys (HNA), that I owe them money from an account that I successfully closed with Telkom over 6 months ago. Telkom confirms that I have no money outstanding. Every time I phone HNA, they say no one is available to speak to as they are all out on lunch. I then ask to speak to the manager, and they say they are only available the next day to speak, and they say this every time I call. They then proceed to hang up on me. Is this company even real? Petunia is the most useless person on their end, as she just hangs up the phone when she doesn't know how to answer the simplest of questions. Is there anyone competent over there to resolve my issue? So far, all I can make out of them is that they are a bogus company.