as a voip engineer - what topics in networking I need to cover ?
You’re going to want to have a pretty good understanding of computer networking as a whole.
VoIP uses all the same infrastructure and protocols that computers do, so you will want a solid understanding of routing, packet switching, subnetting, and firewall configuration.
A good place to start would be to look into the CCNA certification.
I’d say if you understand the traffic and how it traverses each hop then you’re off to a good start. Starting with layer 1, how the VoIP phone connects to the switch, from there to the firewall/router and how it reaches its destination.
Basic networking like routing & switching, VLANs, QoS, NGFWs, TCP/IP stack, and how to read a packet capture. VoIP is just an extension of the network so having a good understanding of networking will help you tremendously.
VLANs, QoS, LLDP-MED, SIP
TCP/IP protocols