Interesting but confusing. If they are calling outbound into SA, what they are doing does not make sense, as it *sounds* like they are using VOIP to carry the traffic to another location from where they must break out to the normal networks. Why not just route voice straight over the links - it will cost less to Vodacom MTN and Cell-C, though maybe not to landlines, as you do not have to pay for the VOIP traffic before breaking out to these networks? Probably cheaper to do this for national landlines only, but a bit of haggling with V,M&C and one of them will give you a good rate into Telkom's landlines nationally.
Or do I misunderstand?
I agree with Taranchio - the call centre I am involved with dials all SA numbers over the GSM networks and has done for a while. We do keep some Telkom links for redundancy though.