If you have a VOIP phone service at home, your service provider will give you an analog telephone adaptor such that you can connect the internet-based (digital) service to your existing (analog) phones. The adaptor will have an IP address just like other devices in your network when plugged into the router.

If you log into the phone adaptor (by pointing your browser to the adaptor’s IP address), you will find tons of configuration that can be tweaked to make it work better (or break it if you don’t know what you are doing). If you are really interested in understanding what or how each parameter should be set, these documents (#001, #002) will give you some clues (for Grandstream adaptors HT70x series).