Network engineering, what exactly does this mean?
What it means is, you simply do not just plug equipment in, and presto, everything is great.
There is much that goes into VoIP network engineering. The statement is very broad, but encompasses many aspects of networking. It requires experience and knowledge. Many people read forums and blogs, and find the same information time and time again. Discovering new buz and key words, and feeling they have found the answers. There are a lot of individuals putting information down, but have actually no working knowledge or hands on experience implementing the things they write or read. This in turn creates confusion.
It is really important to understand, there are many aspects of correctly engineering and installing a VoIP enabled network. If it is improperly engineered, you will experience poor quality and choppy phone calls.
Also, splitting the data and voice network is only necessary, if you cannot figure it out. So if your vendor is telling you to buy another internet connection for VoIP, there's your sign.