How do phone-line network cards work?

A friend asked if there was a way to network his home to his office. They are three blocks apart and do not have a clear line of sight. He needs to have around a 1 Mbit/sec connection, so I think all wireless is out. He does have off-premise extensions of the same phoneline at all three locations. I wonder if he could use phone-line network adaptors like <A HREF="http://www.3com.com/products/en_US/prodlist.jsp?tab=solution&pathtype=purchase&sol_id=1000005&family=411&cat=20&solcat=Home+and+Home+Office" target="_new">the 3com phone-line adaptors</A> ?

How do these work? Do they need their own "phoneline" or can they work over an existing active phone line? What happens with the dial tone, etc?

Thanks.