Use my Land Line with Android? Is it possible using a computer and WiFi, hardware plugin box or something to allow me to make (and receive) telephone calls using my Android phone through the existing land line?

It would be more convenient if I could carry one phone and receive both home phone and mobile phone calls. Also I want to make outgoing calls and have it use my home phone connection for the reduced charges and the correct caller ID.

I'm OK with using WiFi LAN (VoIP), but I don't want to use the internet connection, I want it to go through the hardware phone line that the phone company put in. Is there such a solution?
The Answer to the question is Yes and I do it.

I use a Raspberry Pi running RasPBX so within the house most things are SIP or DECT. My Main phone is a Gigaset DX800A which supports SIP and also DECT. These DECT phones attached to the DX800A become SIP extensions. The Phone in my office is an Aastra 6739i (SIP) with a Sennheiser DECT headset. I have a Linksys SPA3102-UK which connects the PSTN Landline to the Raspberry Pi. The Mobile phone can run any SIP software and become an internal extension, and thus make and receive landline calls.

As to being a workable solution then it depends on your cellphone coverage and how you use your cellphone (do you need SMS?). Another useful configuration is the DX800A will allow you to connect to you Cell phone via Bluetooth and make a receive calls from the DX800A or any DECT handsets attached to it. So in this situation you carry around a DECT handset and you can make and receive calls from both the Mobile number and landline number.

I would like to find an app that converts the Mobile into a SIP/GSM gateway such that the Android phone can be connected to the Asterisk based RasPBX and be used to make outgoing and incoming calls via GSM. This way I could use my calling plan to make calls from my SIP based Desk phone going out via the mobile (routing controlled by RasPBX). But don't know of one as yet.