Internet sharing problem

[jma-kinho]

I am using the Linux Mint 19.1, kernel 4.15. I connect to internet using wifi, and will share the internet using ethernet for a IP Phone Grandstream.
And I have done:
click> network connection > wired connection 1> edit> ipv4 setting >automatic dhcp > change to> shared to other computer>
click> save.
On the kernel 4.4, the conection is ok. But, since the kernel 4.8, the phone can make calls but I dont listen the person on the other side. I don't receive calls too.
I would to know if there is some port to open or something on the iptables.

Thanks for the help

[AndyMH]

In case nobody else responds, you could install the 4.4 kernel. It is an LTS kernel so supported for some time. If everything else works with 4.4 it is a solution.

[jma-kinho]

Unfortunately with the 4.4 I have problems with HDMI audio on the machine that I use. So I need make work with this kernel. I need to know if there was changes on iptables rules on the kernel update or something like this.