I want to set up a home network. I have an extensive unused phone line coverage. should I use these or should I invest in proper networking cable?

I don't understand what you mean by "I have an extensive unused phone line coverage."
Do you have BB or dial up?
How many pcs do you want to network?

I have unused telephone points, I am on BB and wish to use three pc's

I'll ask a few more questions.

1. What sort of budget do you have for this?
2. Do you want all the PCs to independently have access to your BB connection as well as network with each other?
3. Do unsightly cables annoy you? (or possibly your partner as is the cae with me )
4. What type of PCs do you have? (laptops, desktops etc. old or new)
5. What sort of BB connection doy uo have Cable or DSL based and are you in the UK?

The three options I would advise are wired network, wireless network or power line network but I need to know the answers ot the questions before I can give a well considered answer.