Using european telephone cables for own DC power delivery
POE actually uses AC power (the cheap injectors us DC but they are not to spec). Since you don't know what kind of cable you are using, DC could be a problem for three reasons. 


Since you don't know what the wire is, and most of us can't guess what European phone cable is like. You need to worry about the size, you could burn the wire up, you need to figure out what the conductor is rated for, if you can determine the size then you could compare that to a wire gauge chart to get a current rating.
You may have too much DC resistance. Take the wires and tie them together on one end, then take your meter and measure the resistance. If it was more than tens of ohms that is going to be a problem. Lets say you power a PI, they run at about .350mA max plus some other stuff so 1/2 an amp total. You step the voltage up to 48V and then down at the end of the cable which we'll say worst case is 100ohm. 1/2 an amp @5V is 2.5W. 2.5W running at 48V is ~50mA (Plus a little more from the inefficiencies of the DC/DC converters, but we are going to ignore those at this time). 50mA in a 100 ohm cable will be (P=I^2*R) a quarter of a watt which is probably fine. If there is a different signal or voltage that you'll send down the cable use the same process using the rules (V=IR , P=IV and P=I^2*R)
Leakage current through the insulation. You need to make sure your not putting power on anything else and that the insulation can hold 48V. I don't think it will be a problem but its worth mentioning.


If you use the real POE (AC) , it would be better if the phone cable was twisted pair, otherwise the AC might radiate out a bit.