My understanding of this is that they will activate the second pair that is commonly used in the RJ-43 port in your wall. This will allow 2 lines active to the device.

Changes inside might need to happen if your residence does not have 4 wire (2 line) compatibility. (IE 2 pairs to the jack vs 1 pair)

On Thu, Aug 6, 2020 at 9:10 PM Jim via PLUG-discuss <plug-discuss@lists.phxlinux.org> wrote:

Where I live, I get AT&T for my DSL service. I've signed up for an
upgrade from 10 Mbps to 25. I finally got someone there who would tell
me why a technician visit is required for the upgrade. They're bonding 2
pairs to supply the faster speed here. I've read up online about DSL
bonding. I understand that one pair will carry some of the data, and
the other pair will carry some. But one thing I didn't find out was
whether or not anything will change between the wall jack and the
modem. Is everything done outside or do they have to come inside? I
currently have a 2 conductor cord connecting my modem to the wall jack.
Will that have to be replaced with a 4 conductor cord? Do they install
an extra box outside or inside? I guess all will be answered on the
18th when the guy is scheduled to be here. I'm really curious how this
works.
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