The Neotel offering is a wireless service, but Neotel has made a point that it isn't a mobile solution. It's portable - but that portability is limited to the city in which you live. That's because your Neotel phone - which is also the modem for its service - is issued with a geographic number (011 for the Johannesburg area) and trying to use it in Cape Town would probably create havoc with the network. However, its voice quality is excellent and in preliminary testing - even with the wireless signal on one bar - there was no degradation of quality.
I seriously doubt that, overseas companies like Vonage manage to let you do this with no issues, from their perspective it makes sense because they'll probably make more on call charges and interconnect fees that way but I doubt there's a real technical reason.