RJ-45 used with a telphone

I came across a desk phone with a 6 pin connector plugged into an RJ-45 jack. The RJ-25 plug pins line up with the RJ-45 jack, but is this a legitimate practice?

Was it working or did someone just plug the (PBX) phone into a network jack.

Sounds like a multi-line phone.

But, who has phone service on an RJ-45? If you plug it into Ethernet, it’s not going to work…

It had been working, and was reported non working this morning.

People with VOIP phones.

As ninja’d* by boffking, maybe a VOIP phone? Maybe it’s really an Ethernet after all?

Alternately, since terminations are standardized and the contact spacing and placement are compatible, the RJ-45 jack and plug may be terminated to emulate standard RJ-11/RJ-14 contact placement. It would be non-standard (since the largest connector specified for POTS is multi-line analog phone via 6-pin RJ-25), and it would be at risk of being confused with computer networking, but it would work.

*in the 20-minute interval in which SDMB’s CDN and content servers were puking all over themselves, and the only response I could get while posting this was CloudFront errors and 503 codes.

ISDN uses RJ-45, as do VoIP networks (as mentioned).

eta: The building may have been wired for ISDN but downgraded or never implemented. Or maybe the cable installer couldn’t find an RJ11 plug for that jack and just phoned it in that day.


ISDN what you did there.

This is fairly common in the uk. An office will be flood wired with rj45 sockets which can be configured either for phone or network. It’s just a case of patching it in to the correct system on the patch panels. I’ve seen this going back to the mid 90s and that wasn’t voip or isdn, just simple analogue phones. The phones came with an adapter to convert the rj11 to an rj45.

In a good office, the ports will be labeled as phone or network. But in most offices you have to just guess!

Also, I have done some cabling work and if you don’t hook up all the pins in an ethernet cable you can use it as a phone line. RJ-11 has 2 twisted pairs and RJ-45 has 4. If you hook up two of the pairs, and cut the ends off the other wires, it makes a perfectly good phone line. Of course, this takes more time and money then using Cat3 in the first place, but the company I worked for did so much voip work they didn’t keep Cat3 on hand.

Sure, as long as the leads on the jack line up. Assuming this is a 1FB or POTS line, it happens all the time where someone uses CAT3 cable with RJ-45 connectors and plugs their RJ-11 phone jacks into it. Personally, I wouldn’t do it that way (i.e. I’d break out the pairs from the CAT3 or whatever and put in the proper wall jacks), but it works well enough as long as the leads are touching right and the cable isn’t a cross over or anything weird.

Of course, as mentioned, it COULD be a VoIP system, but then your RJ-45 would be a network connection plugged into a switch somewhere (possibly with POE)…and in that case the phone jack wouldn’t be RJ-11.

I’ve used network cable to make phone lines. :slight_smile:

I took the plate off. It is network cable, four pairs of twisted pair, configured as “B”, and runs to the telephone box. We only have two phone lines in that building, already used. I suppose it is configured for when we go to VOIP, God help us, but then why terminate in the phone box instead of a punch down panel for a network switch?

Not surprising.

Its NOT legitimate practice.

There are adaptors available to make it good … some just fill the 8P (8 position) socket to be just right for the 6P plug (holding the 6P straight and firm, and protecting pins 1 n 8 and the rest too… ) Other adaptors are a 6P socket with 8P plug…
Without the adaptor, pins 1 and 8 of the 8P are being used to hold the 6P plug steady and straight and pins are really not meant to do that job.

Two results occur

  1. The 6P can wobble around and bend any of the pins… eg the ones for phone use…
  2. pins 1 and 8 become bent and not usable for ethernet

That was my thought.
I put together a cable and an RJ-45 socket and put my voltmeter to it. Everything works. There is a slot in the socket and a wedge in the plug that help keep things in the right place.

I have untrustworthy sources on both sides here, but I think this was installed for a phone before it was realized that we have only two lines in that building, and the user plugged a phone in believing that we would somehow magically “make it work”.