VoIP is fine - been using it and selling it for years. What isn't fine is the service providers of today trying to dumb it down enough for most non-technical users to be able to install and use their offerings, or pretending it works over any IP network - it doesn't, it needs a certain quality of service (QoS).
Now, while today's broadband services will provide that most of the time, it isn't guaranteed and is "best effort". So, mostly it works, but expect some drops and some cellphone like reception. No way, IMHO, should it ever be sold as a replacement for your Telstra line! It's a brilliant way of getting a cheap extra phone number and cheap STD/International, calls, but it isn't something I'd use as my main number!
However, for cost vs. performance, I wouldn't be without it. I've had Pennytel since I've been in Aus. 8c flat rate for calls interstate and to most country's I want to dial, and an extra Perth PSTN (Normal, public telephone network) number for a $10 deposit! - I'll sacrifice some quality for that any day. 
That said....
Don't touch Engin with a bargepole -as you found out, service sucks, tech support is non-existent except by weekly email exchange, and they blame your ISP for their issues. You can also get cheaper.
Try and get VoIP services from your existing ISP - they can't blame anyone else, and often prioritise their own traffic at least until it exists their network.
Use a decent router, and if you're not VoIP/IP savvy (i.e. If you don't understand SIP, QoS, traffic queuing, latency, and how to troubleshoot all the above) it's worth paying whatever premium and getting the router from the ISP pre-configured. It will save you headache after headache when something goes wrong.
Jeff
1994 300TDi Defender
2010 TDV8 RRS
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