Ended call still rings on receiver devices

[casi-online]

Hi everybody. im totally noob in ip-phones...

setup:
Im using T46s connected to a fritzbox 6591, gigabit cable Vodafone germany.
In my LAN are a pi-hole as DNS and a firewalla blue as security system. Both are more or less stealth, i dont think the Yealink does recognise them in any way....

problem:
When i call any number (mobile, or other landline telephone numbers), and i quit the not established call, before the called user answers, the other device keeps ringing. it doesnt end when i hang up my phone...
Same when i internal call my second handset (DECT connected phone). But not whe i use second handset to call the Yealink. So it is a problem FROM yealink TO all other devices.

Incoming calls are working fine.
I searched the internet, but all answers to this "keeps ringing problem" or setups are really different from my setup, afaik.

I use portforwarding, also the ports were opened via upnp already. 5060+5061
The SIP-Login is done by my fritzbox, so i only entered the telephoneuser in the T46s menu.

Thanks for any suggestions.

Casi