I have limited access to the internet at the moment, otherwise I'd try to give a better answer, but I recall some technology called femtocells, which if I remember correctly, allowed users to receive calls over wifi, particularly in areas where there is little cellular availability.
I have a Vodafone SureSignal (i.e. a femtocell), from the days when I was on Vodafone and couldn't get a signal (in central London). I don't use it now because I am no longer on Vodafone and anyway have a good signal, but it did work well. It wouldn't do what you originally wanted, but would allow you to "just use your mobile".
Yes; a good point. I actually bought the Voda Sure Signal box but couldn't get it to work.
It's installation requires a router which supports a protocol that allows an internal app to open whatever ports it wants, which one obviously doesn't allow in any half secure scenario. My router doesn't support that anyway. So I searched around to find out what ports needed opening manually, without success via Voda's atrocious call centres. I did eventually find out what looked like the required ports but it still wouldn't work. So I sent it back.
I will probably try a proper VOIP app as it should work elsewhere, and also on 3G.
My experience with skype, in different parts of the world, is that it only depends on the quality (the latency) of your network. With a reasonably fast and low latency network, you get perfect connections. Typically way better than a mobile connection. Are you sure your internet connection will be able to cope with any kind of VOIP?
448kbits/sec up, about 6mbits/sec down. Should be enough.
On 3G, in the UK one has about 40k up and 200-300k down.
On HSPA, again in the UK, I get about 1mbit/sec up and 3mbits/sec down. But HSPA is far from common, and it crashes the 808 phone 
Suggest you try TU Go the new iphone / ipad app from O2 . This uses VOIP technology and only requires a WiFi connection. Calls and texts are supported and the receiving party will see it as coming from you mobile. Of course you do need to be on O2 though ..!
I'm not very knowledgeable in this field, but is this similar to what's been talked about:-
I have an htc Sensation MOB on the Orange Network (EE). The signal is poor in the house. I don't know if the following is an Orange feature or an htc feature. But the mobile has a thing called 'Signal Boost'.
Once turned 'on' it stays 'on' (or should do - occasionally it hiccups), but if the MOB can't get a network signal then calls seamlessly come and go via my radio router. Prior to having this feature (a previous MOB) I would be hanging out a window to use it, now the htc Sensation works all round the house - so long as it has WiFi contact with the router.
It works (I think) with any, or at least some other WiFi net works, when an 'EE' signal is weak or not available.
The calls are still mobile calls (not land-line) as Peter wants - but a good system.
Peter,
It depends what you want to achieve.
you can convert your analogue home or office signal to sip and use any sip client.
You can also move your telephone line to a hosted voip solution . In that case you can acces youre line with any sip client on your phone.
You can also use skype to make outgoing calls to the public network.
And last you can use Ms Lync .. with the 2013 version you can make calls through federation with other parties.. As of this summer you will even be able to federate to Skype users.
It's actually an interesting point whether VOIP on a mobile phone is cheaper - if you are on a contract.
Non-roaming, you normally get more free minutes of voice calls than you can use up 
And using deals such as Voda's Euro Traveller, you can take this allowance with you to more or less anywhere in Europe (even outside the EU, e.g. Croatia) for £3/day.
It's true that this applies also to mobile data allowances, which is what VOIP would be running on, but which would you use up first? 100 minutes of conventional voice calls, or 250MB of data making VOIP calls? When using either will cost you £3/day anyway.
Obviously if you bought a local PAYG data SIM, say €10 for 1GB, there is no contest. You go for VOIP. Especially as sticking that SIM in your phone trashes your normal phone number
There is some sort of SIP config in my phone (Nokia 808) so I will investigate.
the markets are moving to flat fee for fixed and mobile.
So in the end you will not gain much with voip over traditional.
Internationally can be different though.
