Εδώ:
http://boards.straightdope.com/sdmb/...d.php?t=405559
λέει αν δεν είναι σωστή η πολικότητα θα λειτουργει το τηλέφωνο
αλλά δεν θα λειτουργεί το τσιπακί για τους τόνους dtmf όταν θέλουμε
να πάρουμε τηλέφωνο:
"The last time I worked for a phone company--some years ago--yes, the polarity does matter. But there's an easy way to check.
The sound generator built in to the touchtone pad requires that the line voltage have the correct polarity, that is, it needs a positive voltage on the positive terminal and a negative voltage on the negative terminal.
If you have the line reversed, you'll hear the dial tone when you pick up and you can receive incoming calls with no problem...but
the touchtone pad won't generate tones.
So, attach the line any which way then try to make a call. If the touchtone pad works, you're fine. If it doesn't, reverse the line connection."