VoIP system stands for Voice Over Internet Protocol. It is a communications technology that delivers voice and multimedia (such as video) over the internet instead of over a phone line.
Examples of VoIP are the excellent 3CX Phone System, and Microsoft Teams. Any device that has the right software can be configured as a VoIP phone. The vast majority of VoIP systems will use the industry standard SIP (Session Initiation Protocol) to start and end their calls. In theory, any SIP phone can connect to any SIP compatible phone system.
If you don’t already know, in the UK parts of the old copper based telephone network are being turned off and the operators are moving to IP (internet protocol) based communications. For businesses this means the ISDN is out and VoIP is in.