To set up a local network in your office, you will need networking hardware.
If the Internet is to be connected to the server directly (installation of two network cards in the server is required) then you only need network switch.
The number of ports is determined by the number of devices in the local network.
You can also use the router, in this case, the server will act as a client (only one network card will be used), and the router can organize a wireless Wi-Fi network (if there is a Wifi module in the router), but in this case it will be necessary to have Wifi for normal operation of the server.
You may choose Mikrotik router, but setting up might not be easy.
You will also need switiching cable to connect computers to the local network.
There are several types of voip gateways. The most commonly used are GSM Voip Gateways and analog voip gateways. GSM gateways are used to be able to make and receive calls from cellular lines.
Analog gateways are used to connect city analog lines. You may also need equipment for connecting the E1 line (the best option is the PCI card).
`Warning! When using an analog line, reliable detection of network signals (busy, answering and others) is not guaranteed. In this case, it will not be possible to set up the automatic call back to customers.
In the case of connecting the telephone line via SIP (Voip telephony) - additional equipment is not required.