The SPA-3000 will try to detect disconnect from PSTN line by way of polarity reversal or disconnect tone (defaulted to US PSTN spec).

If the above is failed to be detected due to special connection to devices such as PBXs or connection to other coutries PSTN , then the disconnect will fallback on "Long Silence" timer value.  Alternatively, you can force disconnect it.

If you are the caller (PSTN caller or VoIP caller), and the call is not a ring-thru-Line-1 or forwarded call.  You can enter **# to force the SPA to take the FXO port on-hook to disconnect the PSTN line.