VOIP worked with Mint 19.1 & kernel 4.4, fails w/ kernel 4.8

With Linux Mint 19.1, kernel 4.15, I connect to the Internet using WiFi, and want to share the Ethernet connection with a Grandstream IP Phone . I have done: > click | network connection | wired connection 1 | edit | ipv4 setting > | automatic dhcp | change to | shared to other computer | click | save With kernel 4.4, the connection is OK, but, since changing to kernel 4.8, the phone can make calls, but I don't hear the person on the other side. I don't receive calls, either. I would to know if there is some port to open or something to do with the iptables. Phone Model: Grandstream GXP1625.