Telephone lines are long copper wires from central office to your apartment. Usually those wires are between few hundred meters to few kilometers.
Those wires are connected in the central office to the telephone center/exchange. Normally there is only the 48V DC voltage from central office battery in the line, and when phone ring there is 70-130V AC voltage in the line. That’s what in the line in normal operation.
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But the situation in the thunderstorm is different. When the lightning hits the ground with it’s millions of volts and thousands of ampreres strike every wire in the ground or hanging in the air will notice it. Near lightning strike gan generate a surge pulse of thousands of volts to telephone lines. If this can go to the electronics inside the telephone or modem, the electronics will destroy at the same moment.
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