Hello Amir35357 and sudogo,
If I were you, I won't trust any of the above answers.
The thing is, there is no way that you can use HG8245H's TEL ports to connect to a local VoIP server that hosted on your LAN side. Yes I acknowledge that there is a "VoIP interface" parameter available for you to choose from a list of interfaces. Unfortunately, these so-called interfaces are all virtual interfaces that bounded to your upstream (WAN/PON) port. Nothing is programmed at the factory that allows you bound a VoIP interface to your LAN side. VoIP servers must at WAN/PON side.
If you really need these POTS (TEL) ports for your LAN side VoIP server, do not even think about using HG8245H. Buy a FXO/FXS instead.
Answer from an unimportant Huawei customer.