Thanks for all the suggestions. Let me clarify what I am thinking....
Yes, this is from anywhere (so Wifi isn't the answer). I'm over the other "pond" from you! In Australia there isn't any distinction between distance for mobile calls, they are charged by the minute whether the person is next to you or on the other side of the country, so that isn't the issue.
As I mentioned above, I am trying to avoid mobile data, and thus a SIP connection, because this is charged for, and (although I haven't checked the plans recently) I suspect that the equivalent data charge for a minute of data even with a good codec is likely to be more than a minute of normal voice time, so there would be no advantage.
I was thinking about picking up one of those voice plans where you can specify one or more fixed line numbers to be freecalls (like a "family and friends plan"). If that was my inbound VoIP number to my Asterisk server, then I could place freecalls to it all the time. Then I can get a VoIP plan that is very competitive compared with mobile plan minutes, and use that to route the calls back out again. Thus it needs to be a voice call to the Asterisk box then a SIP call out, with something to coordinate rerouting calls this way on the mobile phone and on Asterisk.
Was just a thought... doesn't sound like anybody has come up with something like this yet. Thanks anyway!!