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Warning Some Countries forbid connection of non-government approved telephone equipment OR tampering/tinkering with telephone line
** * If info is for educational purposes only * ** ***
1st Understand the concept of phone not in use and hung-up.
How do you detect a phone is off hook.... what happens to the electrical signals? Ask yourself those questions.
When the phone is on-hook (hung up) the phone has a higher DC resistance... as if it were not connected....
When the phone is picked up a lower DC resistance is applied by the hook switch to the phone line.
The lower DC resistance has 2 effects
Draws current from the phone exchange typically 20mA to 90mA depending on how far from the exchange
The voltage drops from typically 48 volts to typically 6v ~ 18v depending on how far you are away from the exchange.
OK.... you how have two effects you can measure....
current ... which requires a series in-line device
voltage.... measure the voltage across the phone point.... parallel circuit.
NOW.... when you measure electrical signals you should not effect the item you are measuring.
So a series current or parallel voltage measurement is required.
Note... there are other complex factors involved..... but this info may partly explain the DC characteristic of phone and exchanges.
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