This seems like the solution, but as some have pointed out BEWARE of scam calls, so i'll have to stick with more technical solutions.Use 2 of these (https://www.vikingelectronics.com/product_docs/DOD/406.pdf) in parallel.
Use terminals 6 and 7 of First to trigger your system
Connect 6 and 7 of the second to the telephone Lines.
When the Ist Ring is received, both the relays operate, 1 triggers your system, the second extends a loop to the exchange to trip further rings.
This will activate the relay after a single digit is pressed, (that what i understood, that it doesn't activate after multiple digits are pressed,, it uses just a single digit). Hence making it easy for anybody that knows the phone number to guess the digit.
What's a UP?Alternative is to use a UP to detect ring and decode tone(s).
These 2 seem like the most correct solution, but I have zero knowledge with arduino.Here is another possibility - http://www.cermetek.com/catalog/telephone-line-interface/datasheet/xe0068dtrevc.pdf
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by Jake Hertz
by Jake Hertz