Are you sure you were using the RJ45 port on the Huawei device? From the specifications online I can see, that it also has an RJ11 port for telephones.
Since the RUTX11 does not have such a port, nothing needs to be configured on the RUTX11, and everything should be done on the third-party device.
You will most likely need to configure port forwarding to the device in question. To do so, navigate to Network → Firewall → Port Forwards. Here, create a new rule with a distinctive name, and forward the needed port to the LAN IP of the third-party device. The port(s), that need to be forwarded could be found in the third-party device settings. For VoIP communications, the most commonly used port is 5060.