Mixing connectors is a bad idea. Imagine some idiot plugging a VOIP phone into a conventional phone wired outlet because it used RJ45 instead of RJ48 and you begin to understand why the distinction is important. Ethernet equipment typically isn't designed to handle the 90+ VAC ring voltages of a POTS line. Gigabit Ethernet voltages go as high as 4V.
Many PBXs are made to work on standard phone lines, but most commercial and even some residential guys are just opting to use all RJ45 jacks so they do not have to stock/carry both. Also many home phone blocks are starting to use RJ45 for the jumpers so they can be dual use (like the structured boxes from leviton, etc.)
Then there is the separate issue of PoE that may find its way up to 57VDC.
Know that if the connector fits, someone somewhere is going to plug something into it that it wasn't anticipated. If you do just a little research, you'll find out quickly why they stopped using telephone handset connectors for keyboards.