("Newps" wrote)
It takes two wires to have a voice line. There are typically 4 wires in
a residential phone line; red, green, yellow and black. Normally the
first phone number uses the red and green wires. The other two will be
for your second phone number, should you get one. The DSL uses the same
two wires as your first phone number, with a filter to keep the two
things separate.

Looks like a clear explanation. Thanks.

--
Montblack