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That's not how it works with digital lines. Disconnect on any side breaks the whole circuit presuming they conform to even ancient PDH, much less SONET or SDH. Oh and this includes even more ancient ISDN. The trunk will immediately tear down the DS0 slot and circuit.

GSM and VoIP also do not allow this behavior without engaging call waiting on subscriber side.

This only happens with old fully analog connections. Not sure which country or public operator still has this kind of PSTN.

For the difference, peruse ITU-T G.175 and Q.522 standards. SE (switching element) will disconnect the routing on your side. It took me a while to find the actual standard number.

I haven't heard of this working since the 80's, with rotary pulse dial phones.

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