SUMMARY:

https://www.instructables.com/id/Hack-a-VOIP-Box-In...

The above instructable tells you how to use a Linksys VOIP box to connect to POTS phones together and make a sort of simple home intercom system. However, many people (myself included) wondered if additional phones could be added to this system if 2 or more VOIP boxes were on the network.
After some experimenting I was able to make this happen on my home network using a Linksys SPA-2012 and a Linksys SPA-1001, which gave me 3 phones that can all call each other.
Additional VOIP boxes could also be added for more phones.
Also, the SPA-1001 is not only connected to the home-network, but can also call out via my CallCentric account (or any other VOIP account..I just happen to use CallCentric)

THE PROBLEM:

I wanted a simple phone-system running from my kids treehouse to a little playhouse in the back. I had some basic phones I found at thrift-stores. I read various instructables telling me how to add a small bit of voltage to the phones to make them operable; however, this method did not have a way of making the phones ring.
Other people had some complex methods of adding a button connected to ringers and such, but nothing suited me.

Finally, I stumbled on this instructable:

https://www.instructables.com/id/Hack-a-VOIP-Box-In...

I happened to have a Linksys two-line VOIP box sitting on the shelf, and dinkered around until I was able to get things working. And indeed it worked. The two lines were able to call each other, and I could program the dial-plan for specific keys (press 1 to call treehouse...press 1 to call playhouse)

Wonderful, but then I wanted more...I wanted my separate VOIP box in my work-shed to be able to call the treehouse and playhouse...and for them to call my shed, but I still wanted to be able to use my VOIP service to call out for long-distance.

After much experimenting, I was able to make it happen. Here is how:

Step 1: First Step: the Right Kind of VOIP Box

I mentioned that for this project, I originally started with this wonderful instructable:

https://www.instructables.com/id/Hack-a-VOIP-Box-In...

My instructable is simply an addition to this

The only difference was that I used a Linksys SPA2102, while he used a Linksys PAP2T; however, the differences were few and I followed his instructions and things worked.
If you browse down to the comments on that instructable, there were some tips for dial-plans to make specific numbers for each phone. I followed those and was able to make 1 the dial-number for each phone to call the other.

The dial-plan is the very last field in the form to fill in. The code I ended up with looked like this:

(<1:> S0 <:treehouse@127.0.0.1:5061>) this goes in the dial-plan for the playhouse

(<1:> S0 <:playhouse@127.0.0.1:5060>) this goes in the dial-plan for the treehouse

see the screen-shot for a closer look.

Okay, so that gave me two wired phones with a VOIP box in between them that can call each-other by pressing 1.

Now I want to add another VOIP box in a different location on the property (my work-shed) that is connected to the same network, and can call out, but also press certain keys and call the playhouse and treehouse.

The VOIP box I use in my shed is a Linksys SPA-1001 (unlocked, of course). This little device is nifty because it not only can turn your regular POTS phone into a VOIP phone, but it also magically adds a second line to a single-line phone.
I had already set up the first line to my CallCentric account, using these instructions:

http://www.callcentric.com/support/device/linksys/...

The second line I had forgotten about completely until I started trying to figure out a way to connect to my little home-phone-line system. Then it dawned on me that I might be able to make the second-line on this device the home network line.
With a little tweaking, I was able to do just that!

Step 2: Line 2 of the SPA-1001

So my SPA-1001 is connected to my router, and configured on line1 to use CallCentric for cheap long-distance phone-calls.
I want line2 to be connected to my home-phone system, so the kids can call my shed from the playhouse or treehouse.

So just to clarify:

the treehouse and playhouse are both connected to the SPA-2102
the SPA-2102 is connected to my home-router and has an IP address of 10.1.0.102
the phone in my shed is connected to the SPA-1001
the SPA-1001 is connected to the same home-router and has an IP address of 10.1.0.103
the IP address does not matter as long as it remains the same for each VOIP box
Line1 of my SPA-1001 is configured to work with CallCentric

First thing I did was go into the SPA-1001, under the admin user, on the Phone tab.

Default line is set to: Line 1
Line 2 Select Code is: #

Next I went into Line 2 tab. Set up is identical to configuration at:

https://www.instructables.com/id/Hack-a-VOIP-Box-In...

The only difference is the dial-plan. Here is what works for me:

(<1:> S0 <:playhouse@10.1.0.102:5060>|<2:> S0 <:treehouse@10.1.0.102:5061>)

on the phone I have to first press: #

This switches me to "line2", and I can dial 1 or 2, which will call the playhouse or treehouse.

Next, I have to change the dial-plans for the treehouse and playhouse

Step 3: Dial Plans for Treehouse and Playhouse

Finally, I add the shed to the dial-plans for the treehouse and playhouse:

Playhouse dial-plan:

(<1:> S0 <:treehouse@127.0.0.1:5061>|<2:> S0 <:cottage@10.1.0.103:5061>)

Treehouse dial-plan:

(<1:> S0 <:playhouse@127.0.0.1:5060>|<2:> S0 <:cottage@10.1.0.103:5061>)

and that does it. Now all 3 phones can dial each-other, and my shed phone can still call out. I might buy another SPA-1001 for my wife to have a land-line that can call all three phones.

If you had property with several buildings, or rooms that were far away from each other, but the same home-network, you could easily create a home-phone system for each building/room. Unlocked SPA-1001s are cheap on ebay.

Someone also suggested that these phones could be made to call remote locations that were connected via VPN or port-forwarding. That would be kind of cool to work out because it could be a way to create a personal phone-system that extended to family and friends at various locations.