[H] Phone-internal?

[DSinc]

I now appears that my homes internal phone wiring has died. I have to 
replace it.  I remain active via a long "phone cord" thru a window to 
the TSID (NID box).

What I read says I need to request Cat5/Cat6 replacement wiring. OK!
Understand.
Believe/think my current wiring is old 4-wire single copper line. I have 
read up about the "cross-talk" issues in 4-wire systems.

Question: Is current Cat5/Cat6 internal wire single filament or 
multiple-filament type cable?
Best,
Duncan