Given that it is landlines at work it may hold up on the basis that it discriminates but is not unfair discrimination. There is some logic thought, I.e stability, not fly by night etc.
As an absolute disqualifier it seems absurd, particularly if you consider I can get a neotel pay as you go for the purchase of handset, about R600. I keep my number even if I'm not buying airtime. Pretty easy to stabilise the business.