It is standard "local loop" ("last mile") phone line. So maximum length is defined in the terms of electric parameters rather than meters. It depend on cooper wire diameter, for example.
The resistance of 22 AWG (0.65mm) wire (single lead) is 54 ohm per 1000m, but for 24 AWG (0.5mm) wire is it 85 ohm per 1000m, so 22 AWG based local loop can be about 1.5x longer than 24 AWG based local loop.
With 24 AWG cooper (standard solid UTP CAT 5 cable wire) you should be on the safe side as long as we are speaking about few kilometers (up to 3km or so). If you are going to use UTP cable for it and have no other use for other pairs, you may consider to bind two pairs together (it will reduce loop resistance to half).
Unfortunately, interferences from sources on the path may render the line unusable even for short loop. Also, data or fax transmission over line is more sensitive to noise than old plain human voice. So your mileage may vary.
Don't forget appropriate lightning protection if installed in outdoor environment.
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