Identify source of current in phone lines and reclaim use of phone lines?
Based on the description, it sounds like you (or a previous owner of the house) USED to have traditional phone service and do not have that any more. If you are still paying for phone service then you may want to look into porting the number to VOIP instead of simply turning off the phone service and using VOIP with a new number.
Find the Network Interface
The network interface is the connection from the phone company network to your local equipment (phones/fax/etc.) With newer homes this is typically a clearly marked box with an easy to disconnect connection between the inside wiring and the phone company incoming lines.
In an older house this may be simply a small terminal block &quot;anywhere&quot; with one pair of wires for each phone line coming in and one or more pairs of wires (like the 3-pair UTP you found) going to the rest of the house.
A newer network interface will typically be on the outside of the house or together with electrical and other connections inside the house. An older network interface could be &quot;anywhere&quot;, though near the electrical panel is typical. If your utilities come from overhead wires (aka telephone poles) then you can often trace the wires around the house and look for where the phone wire (typically smaller than the main electrical service) goes into the side of the house and look for a terminal block on the other side of the wall. If you have underground utilities then it will be a little harder to find where the wires enter the house.
Disconnect the Old
Once you find the network interface, disconnect either the incoming (phone company) or outgoing (home wiring) wires. With a modern network interface, it should be very easy to disconnect the home wiring. With a terminal block, it may vary. If you are not 100% sure, take a picture and/or mark the wires clearly so that you can put everything back the way it was previously. If you disconnect some wires and you still get 26V on the UTP then you haven't found the real network interface - but you may disconnected one section of phone wiring from another such that your VOIP would only work in part of the house.
Connect the New
Depending on how the VOIP works, you may be able to connect it directly to the network interface. However, many systems are designed so that you can simply plug the VOIP line into any existing phone jack and all other phone jacks on the house (sharing a linked pair of wires) will be instantly &quot;live&quot;. You can use a line splitter to make the the jack used for the VOIP connection live at the same time.