I have a Gigaset COMFORT A IP FLEX cordless phone connected to the Archer VR2800v router on my landline, so only via RJ11 telephone cable, no VoIP, on analog mode, and my problem is that when the caller on the other end of the phone hangs up, my cordless phone doesn't automatically disconnect as well, but just keeps getting a busy tone endlessly without ever ending the call.

How can I make the phone hang up automatically after the caller on the other end hangs up? 

What systems does TP-Link use to detect the end of a call? Maybe CPC (Calling Party Control)? And how can I change this setting?