If you are using a cloud-based 3CX PBX and you have TLS enabled, you need to make a setting on Yealink phones.
The BLF display will show the colleges, busy or disconnect. The phone will not be able to send a hang-up signal, and you will experience problems with call forwarding.
To fix this problem, simply open the Yealink phone interface and change the following settings as described.
Account > Advanced > Rport (default is Enable)
Account > Advanced > Rport (change it to Enable Direct Process)
You can see BLFs lights as green.
If you want to fix this problem with 3CX Provisioning Method:
Open 3CX Management Console > Settings > Templates > Select your Custom Yealink Template and Search “account.1.nat.rport”.
You will see the following screen and you need to change “account.1.nat.rport” value to “2”.
And now you can reprovision or reboot your Yealink phones.