Their new VoIP phone system installation, all phones with a single run VOIP Service Line

First of all, if you are subscribed to broadband or high speed internet you need to get VOIP - Period!  VOIP in itself is superior to any land-line service, especially in cost, and should be piggy-backed with your broadband line to take advantage of the service that's already there (and that you're already paying for). Most of the potential bandwidth in this line is goes unused, so adding a little more load on that line is insignificant, and free. The cost of VOIP is separate but so is a land-line, and all the advantages and perks that come with VOIP will not be discussed here.

The confusion comes during the installation of VOIP and most new users overlook one of the easiest, yet most useful parts of the service - it can work on all phones throughout the building, not just the one closest (and connected to) the adapter. This "adapter" is actually a router which is shipped and supplied free by most carriers. A router is similar to a multi-socket extension cord. The broadband line that comes from the modem (supplied by the broadband service) simply plugs into the router and then there are multiple outlets from the router where you can run one or more broadband lines to one or more computers or other devices (to form a network).
 
Also located on the router are phone jacks for plugging phones into. Notice the plural term "jacks." This is where new VOIP subscribers short change themselves. The tendency is to simply run a phone line from one of those jacks to the nearest phone and boom, you have your new VOIP. Unfortunately, you're also tethered to just that one phone, or you may decide to run a very long extension line to another phone in another location. Wireless handsets make this set-up a little more convenient and multilple wireless handsets go even further. However, it gets much better than that.
 
You might as well put all your existing phones back to work and for no extra cost - Here's how (this is assuming that you will no longer be using your old land-line service): After the VOIP is installed and running on the phone that's plugged directly into the router, have the land-line phone company disconnect their service line from the junction box on your house (or building that the old service was connected to). The phone company must physically disconnect this line, not just turn off the service and leave the line connected.

Next, simply connect the phone line from the router into the nearest phone jack on the wall, or wherever the nearest phone was plugged in before VOIP. That's it - now all phone jacks throughout the building are live with your new VOIP service (just like they used to be with the old land-line service). The phone that was plugged directly into the router previously can either go back into one of the other phone jacks on the router or into a 2-into-1 splitter where the VOIP line connects to the wall jack. The splitter is usually supplied by the VOIP service with the router or one can be found for a few dollars at any electronics or hardware store.