Because Voice over IP is based on classic IP networks, the installation, preparation and configuration of a VoIP network requires knowledge of IP and network configuration.
Classically, for example, a DHCP server is required for assigning network addresses to your IP phones.
For example, it may be necessary to segment your network or to deal with routing.
Last but not least, the topic of security also plays a role, since the conversations usually leave your own network and connect to phones somewhere in the world.
Firewall, HTTPS, SSL, VPN and Co. are just a few buzzwords we have to talk about in this context.
→ To get a first impression and a brief overview of the topic, please read the article: Bascics of Network