This topic

Can I use an adapter BT431 to RJ45 to connect a BT landline via a Y cable to a router?

richarda697 — Wed, 29 Oct 2025 19:54:21 GMT

Years ago BT upgraded my landline connection to deliver broadband DSL to my router.

I kept my landline connection to legacy phones.

I am in process of getting DECT handheld phones installed to replace my legacy phones, and I need a landline connection into the router to do that.

The 'Y' configured connecting cable from the router to the BT sockets (DSL and BT431) has a DSL plug, and a lead labelled IDSL terminating in a RJ45 plug.

Can I get an adapter to convert the BT431 connection to RJ45? Would the 50V supply damage my router? Or do I need to get BT to change their line termination from DSL+BT431 to DSL_RJ45?

I want to keep my landline connection up and available until I am certain that the DECT phones are fully operational.

Re: Can I use an adapter BT431 to RJ45 to connect a BT landline via a Y cable to a router?

licquorice — Wed, 29 Oct 2025 20:29:49 GMT

You can't just connect an analogue phone line to a router.

When you say DECT do you actually mean VoIP?

DECT are just standard cordless phones.

I have absolutely no idea what you are trying to do.

Re: Can I use an adapter BT431 to RJ45 to connect a BT landline via a Y cable to a router?

richarda697 — Fri, 31 Oct 2025 09:26:45 GMT

Prior to posting here, I tried asking BT directly for help. They tried to answer, but failed.

A bit more information. I hold my BT router as a backup/standby, which means that I use a router of foreign manufacture as my working link to the internet.

I figured that I ought to have a look at the installation instructions for that router. While doing so, within the box, I not only found the instructions for installations but also some adapters, and a couple of cables to allow connection to the UK system. It didn't completely solve the challenge, but I got there.

I had an adapter BT431 to RJ11, and I now had a RJ11 (plug) to RJ45 (socket) adapter so was good to go, and an online check showed that the two pin connections were correctly aligned. I figured that since the manufacturers installation instructions made no mention of the 50V DC issue, I was safe to proceed. (looking back, backing up the router data is advisable though, just in case).

I swapped out the wiring to my legacy handheld phones, updated the data on my router, and made my first call from my DECT phones.

Re: Can I use an adapter BT431 to RJ45 to connect a BT landline via a Y cable to a router?

licquorice — Fri, 31 Oct 2025 10:47:56 GMT

Sorry, that makes absolutely no sense whatsoever.

Re: Can I use an adapter BT431 to RJ45 to connect a BT landline via a Y cable to a router?

gg30340 — Fri, 31 Oct 2025 10:54:15 GMT

@licquoricewrote:
Sorry, that makes absolutely no sense whatsoever.

Glad it's not just me!

Re: Can I use an adapter BT431 to RJ45 to connect a BT landline via a Y cable to a router?

CluelessInNotts — Fri, 31 Oct 2025 22:36:37 GMT

Is your "router of foreign manufacture" a Fritz!Box by any chance?

Re: Can I use an adapter BT431 to RJ45 to connect a BT landline via a Y cable to a router?

richarda697 — Sat, 01 Nov 2025 09:55:33 GMT

Indeed, it is a Fritz!Box.

I had not mentioned that earlier to avoid complicating any discussion.