I'm a network engineer, Cisco certified. I use some of that expertise at my job but I also deal with antiquated phone circuits and equipment like CSU/DSUs. I never worked with the phone system before. Most of my experience is with routers and switches. The phone system is light years easier compared to learning LAN/WAN.
Software engineering is interesting. Besides what I do for my job, I'm also teaching myself C++, Linux Red Hat, penetration testing, and gray hat hacking.