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Long-distance call lock circuit

2017-01-06 13:08  
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Long-distance call lock circuit
The long-distance call lock circuit is as shown, it has the following features: if you want to make a long distance call (the first telephone number is zero), you need to enter the 4-digit password and hang up for 1 ~ 2s then you can make the long distance call; this circuit does not limits the non-long-distance calls (the first telephone number is not zero); you can set any passwords and limit to call the restricted number.