Microwave links are fantastic. Very reliable.
But Vodacom service is ridiculously over priced. Also their customer service is terrible (even at corporate level).
You are going to run voice over IP. A concept that is not new. Over a wireless connection. Not an issue. It works. Weather? I have never experienced any downtime with weather in any of the links that I have tested (wimax, microwave, wifi) and I have had really bad weather a couple of times and the speed was 100%.
If I were you, I should have ported to Neotel or to a decent VOIP telecom. With Neotel you could have a fibre connection (or microwave) to run your data traffic and SIP channels to run your voice.
With a VOIP telecom, you require separate things, but they usually can assist to source the best connection for your requirements. Also you can keep your existing internet connection and use it as failover for your voice traffic.