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Field guide · Voice & telecom

Testing an Analogue Phone Line

How to tell a dead line from dead equipment, and an inside-wiring fault from a carrier fault, using a butt set and a multimeter. Covers on-hook voltage, loop current, ring voltage and the polarity check, what each reading should be, and what a wrong one means — including the readings that say the pair is shorted, open, or crossed with another line.

Diagram

Tip, ring, and the four numbers that tell you whose fault it is.

Line state

Nothing lifted. Battery voltage across tip and ring.

Meter set to

DC volts

Across tip and ring, at the demarc first and then at the jack. Reading both is what tells you which side of the boundary the fault is on.

Expected

42 – 56V

Nominally about 48 V.

If it reads otherwise
Near zero
No battery. The line is dead at the demarc or upstream of it — not a wiring fault inside the building.
Well under 42 V
Something else is loading the pair. Another set off-hook, a bridged extension, or moisture in a joint.
Correct, but noisy
Battery is present and the pair is compromised. Look for a partial short before blaming the equipment.
Fig. 01What the meter should read in each line state, and what every wrong reading narrows the fault to.

Procedure

Doing it

Four measurements, in the order that narrows the fault fastest — and the one that tells you whether to call the carrier.

Tools
  • Butt set with monitor mode
  • Multimeter reading AC and DC volts
  • Toner and probe

  1. Go on-hook first and measure across the pair

    With nothing off-hook, measure DC volts across tip and ring. Expect roughly 48 V — anything from about 42 to 56 is normal. Near zero means no battery from the exchange or an open pair. Around 5 to 10 V usually means something is already off-hook somewhere on the line.

    CautionRing voltage can exceed 90 V AC. Do not hold both conductors while a call is coming in.

  2. Check polarity while you are there

    Tip should read positive with respect to ring on a correctly wired line. Reversed polarity still passes a call, which is why it survives, but it upsets some fax machines, older modems, and coin and message-waiting circuits.

  3. Go off-hook and read loop current

    Put the set off-hook and measure current in series, or read the voltage drop. Off-hook voltage should collapse to roughly 5 to 10 V and loop current should land between about 23 and 60 mA. Under 20 mA and the line is too long or too resistive to hold a reliable call — that points at the carrier or the wiring rather than the phone.

  4. Listen before you conclude

    Monitor mode on the butt set is the fastest instrument you have. Hum on the pair points at a grounded conductor or an unbalanced pair. Crosstalk from another conversation means a split or a crossed pair at a block. Complete silence with normal on-hook voltage often means the fault is downstream of where you are standing.

    CautionBridge in monitor mode, not talk, on a line you do not own. Going off-hook on a live circuit drops whatever call is on it.

  5. Split the building in half

    Disconnect at the demarcation point and test the carrier side alone. Normal readings there and a fault inside means the problem is yours; a fault at the demarc means it is theirs, and you now have the one measurement that gets a carrier ticket taken seriously.

  6. Test the pair itself, not just the line

    With the pair disconnected at both ends, check continuity end to end, resistance between the two conductors, and resistance from each conductor to ground. A short reads near zero between conductors; an open reads infinite along one; a wet or damaged pair shows a low reading to ground and is the classic intermittent that only fails when it rains.

Questions

Questions people actually ask

What should a phone line read on a multimeter?
About 48 V DC across tip and ring on-hook, dropping to roughly 5 to 10 V off-hook, with 20 to 60 mA of loop current. Ringing is a superimposed AC voltage that can exceed 90 V at around 20 Hz. Anything far outside those bands points at a specific fault rather than a vague one.
The line has voltage but no dial tone. What now?
Voltage without dial tone usually means the pair reaches the exchange but the loop cannot close — a high-resistance joint, a corroded punchdown, or a partial open. Go off-hook and read the current: if the voltage does not collapse and current stays under about 20 mA, the loop is not closing, which puts the fault somewhere in the path rather than in the phone.
Does reversed polarity matter?
Not for a normal voice call, which is why it goes unnoticed for years. It does matter for some fax machines, older modems, message-waiting indicators, and anything expecting a specific tip and ring orientation. It costs nothing to correct while you are already at the block.
Is it safe to test a live line?
With sensible care, yes — the on-hook voltage is low. The hazard is ring voltage, which can exceed 90 V AC and arrives without warning. Do not bridge both conductors by hand, use a set with a proper monitor mode, and treat any line feeding an alarm or elevator as one you do not interrupt.

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