This is what I want to do: use my iPhone when I'm at home as my landline handset.
Mobile reception at home is awful so bypassing landline that way is not an option. Also the landline leg of making calls is obviously very reliable (unlike voip).
Using SIP/SKYPE etc is possible but the service is a bit flakey; would prefer to make some sort of hop (via WIFI?) from iPhone to router to make an outbound call (or answering incoming on home phone number).