Hello everyone I set up my 3CX PBX v.15.5 SP2 on a Debian 8. Firewall Checker and SIP-Trunk are configured. Now I got a problem with my Yealink T42G. Required Firmware is installed, SIP Account configured. The number of the extension and the name is shown on the display. Until now, everything looks normal. But every 4 seconds, the phone is unregistered and a second later registered again. During this process, the message "No Service" is shown on the display. If I want to call another extension, there's no sound on the telephone receiver. Same if I try to make an outbound call to an external number. If I use a Snom 320, everything works. Internal and external calls, extension name and number. Same thing with the 3CX Client for Windows. Can you please help me, so I can use this phones and don't have to throw them away. Thanks for your support.