One of biggest arguments against voip is the cost of upgrading wiring to use it. Well I recently discovered a new product made by Phybridge. It will allow the deployment of ip phones over existing cat 3 using one pair. It will allow a distance of 1200 feet from switch to phone as well.
You install the voip "Uniphyer" voip enabler between the data switch the ip system is connected to and your cat 3 IDF using a standard amped 25 pair cable. Then you crossconnect as you normally would either from a 66 or 110 block. At the station end you plug the ip phone into a PhyAdapter (a buscuit jack) which converts the one pair usoc back to ethernet and plug the adapter into the wall jack. No power supply is needed because POE is also provided on the same one pair from the UniPhyer.

You centrally converge with the customer's data network for access to other sites.
The Phybridge provides POE, QOS and a dedicated path for voice with no interference with the data side.

This system is designed to deploy voip phones. It is not designed to run normal office data applications. The bandwidth per port is 175k down and 2500k up. This is fine for one call per port but is not enough for web browsing.

Here is their link

https://www.phybridge.com/uniphyer-ip-phones.aspx

This has been out for a while, it was posted here a few years back...does it work?

I don't know from experience but it looks good to me. I know that a very old upscale Hotel in Va Beach used it to deploy Mitel ip phones to every room using existing cat 3 cable.