Actually, there are several reasons why they may not.
I think you may be surprised as to who drops retail rates, and by how much. Certainly, this whole exercise would have been a waste of time for Telkom's shareholders if Telkom simply passes all the savings on - it would make a big dent in their revenues (making them a smaller company, basically, but with the same high overheads, number of staff etc, something no listed company should risk), and would not improve their profits. Similarly, Cell C would be foolish to pass on all the savings, since they need the additional profit margin just to keep their heads above water.
The big question is the degree of elasticity in the market - how many more calls will each of these networks get to make up the loss of revenue from lower retail rates? Again, I think you may be surprised which companies could benefit, and which not. A typical LCR provider, for example, wouldn't see any particular cost reduction, so is likely to lose customers. Similarly, the mobile operators live and die by their ARPU (average revenue per user), so, if the elasticity is less than 1 (i.e. they don't make up the missing revenue through the same value in additional calls), then they are going to be very reluctant to reduce prices at all. It guess it's a question of who gets brave soonest.
All very true. I think any reductions are likely to initially come from smaller, probably VOIP operators.
High interconnect rates favour the incumbents as obviously they have the most subscribers so they're the largest recipient of these fees and are likely to be net beneficiaries. Interconnect rates also place a floor on off-net call rates which would have an impact on call rates in general and again this benefits large incumbents who will have the most on-net calls as they have the largest subscriber base. Its a classic network effect which will be diminished by lower interconnect rates.
The advantage of significantly lower interconnect rates is that VOIP operators will be able to offer significantly lower call rates to the major plays e.g. Voda, MTN etc. and their own small subscriber base will be less of a hindrance. They're also likely to have much lower fixed costs so they can afford to do this.