@Stillife, in your situation this is what I recommend:
Download and install Axon ( on NCH's site http://nch.com.au )
This software is an advanced call management system (but still simple to use) that can distribute calls across a network to other computers, to digital SIP phones, and to traditional analog phones (using an ATA adapter). (You can still plug your analog phones into your phone jacks as normal, but they won't be connected to the Axon software which allows for much more advanced use of your telephone).
Then you can choose to either
A) Use your current Analog phone line from your Telecommunications provider (will require the purchase of a Linksys/Sipura 3102 FXO adapter, and is costly in terms of call charges)
You can move to a VoIP provider (my personal recommendation. This means all your calls will go over the internet at only a fraction of the cost of your current phone bill. You can even port your phone number over to the VoIP provider so you don't have to go tell everybody you've changed numbers).
The next step is to set-up either the FXO adapter or your new compatible VoIP provider with Axon (by creating a new External line in the software).
Once this is done you can set-up as many extensions as you wish in Axon, and each computer softphone, digital SIP phone, or ATA adapter you have can be associated/registered with an extension.
After setting up a dialing plan in Axon for making outbound calls, and setting up a Group/Queue with all the phone extensions you want to ring on an incoming call... you should then be able to call between extensions in your house as well as receive calls from any of these extensions. Each extension acts as its own "line" and you can have more than one call to different people at the same time (the number of simultaneous calls will be limited by your VoIP provider, and is limited to 1 if you keep your normal analogue land line).
The last step is to download and install IVM (you will need to purchase IVM to continue using it after the trial period has ended). IVM will set-up with Axon, and you can have your incoming calls answered by IVM if there is no Answer on any of your phones.