Vonage looks interesting, but I can't see how it achieves your stated objective of being able to use your mobile through wifi....or am I missing something?
The other thing I have is Skype to Go which allows me to make calls to any phone from my mobile or landline via a local number (which can be changed depending on the country you are in) which costs micro-pennies per minute to a mobile number or is free to most landlines in most countries
I can't see how it achieves your stated objective of being able to use your mobile through wifi.
Not for free (free except that the call would still be potentially billed, being a domestic call) which is what I originally wanted, but presumably one can use one's own home wifi, or any other usable wifi, to connect to a VOIP provider?
We use it for our home number but they also have an app which let you make calls over a WiFi connection. It is very cheap and I have used it from all over the world. Calls to the US for example used to be free - not sure if they still are.
they also have an app which let you make calls over a WiFi connection
Why should there be an "app" for that?
If a phone supports VOIP then it supports VOIP over whatever internet connection is available, surely?
GPRS obviously won't work (~20kbits/sec downlink; EVEN less on the uplink) unless the compression is like the average Indian call centre 
But 3G (100k up, 300k down, perhaps) and HSPA (10x faster again) should all be usable.
Even good old ISDN, top quality voice, runs at 64kbits/sec (8k samples/sec, 8 bits each).
I guess it depends whether their SIP is open access or only open to their apps. Presumably linked to their charging model.
They all use the same compression algorithm. If his quality was poor, it was prob his internet connection.
Isn't it the same issue as with ADSL ISPs?
They actually buy bandwidth from BT, in chunks of X gigabits/sec, and they have to sort out a business model which enables them to map this "per megabyte" charging structure onto the "almost unlimited download data" which they sell to their ADSL customers, relying heavily on most of their customers not going anywhere near the 50GB/month (or whatever).
Surely the same business model will be used on VOIP.
And the point where you make the most money is when the whole setup is just about to fall over
No, the squeeze point on the VOIP pipe is the last mile (for fixed line) or the mobile segment (for mobile). So if you use VOIP in your house, with a poor ADSL connection - it can be poor. Likewise VOIP over mobile even with 3G can be poor - 3G was never set up to work with packetised voice.
Has anyone here actually used Vonage?
I got this from them:
We are unable to share any of our SIP credentials, proxy server address, or other information/assistance that would enable a third party device towards our service. And it is not possible to use our Vonage service through third party SIP phone applications or other VoIP devices. There are few VoIP services provide such option, but our Vonage service doesn't work that way for many reasons.
Our service works either through the Vonage phone adapters or mobile applications which are exclusively designed (basically pre-configured) for using our Vonage service with high digital quality. However, I can realize the benefits you may obtain by having an option to use our SIP credential to configure a third party application. So we are constantly enhancing the scope of our products and limitations. So please check our web sites to see if we've started offering this option in the future.
So they appear to insist on using their app, in an attempt to hide the SIP authentication credentials. I know somebody whose Asterisk server got hacked and he got a large bill as a result, so maybe hacking VOIP ports is a popular hobby - like hacking ISDN PBXs (dial through fraud) has been for many years.
I might give this one a try - it's prepaid so no risk. I have a positive report on it from the owner of the same phone, too.
