I would like to connect my Yealink S20 via a SIP extension of the Fritzbox.
I only found a manual for MyPBX, which doesn't help me for a S20.
Incoming calls will work but outgoing calls will hang for a long time and then get the error "cause 18" in the display of the phone.
I have a working configuration with FreePBX and DrayTekIPBBX behind a FritzBox. So it should work with the S20 as well.