We have the same problem, but I know why. Due to "clip-on fraud" we had to disable the fax line to make any calls where an international dialing code is used. For some reason the 086 numbers appear to be taken as just that and therefore the call can't be made. This "block" is done at the Telkom exchange to ensure that we don't get surprise R 50 000.00 phone bills again. To be able to fax to 086 numbers, we have to take one of the normal phone lines coming from the PABX, plug it into the fax machine, dial 0, and off you go.
Not sure if it might be same thing your side, maybe you could check with Telkom if you have a similar exchange "block" thingy.