Quick question. How come telephone line cannot be used when a
standard data dial up modem is being used and how come telephone
line can be used when DSL connection is active. Thanks.
Simple. What does dial up tells you?
When using dial up, your computer actually calling through your
house line, and data is transfer when the call is connected, and
line will be only dis-connected when you decided to end the
session. So is like, you're talking though phone, to collects data
from your friends, but this time, is your computer who is doing all
the stuff and hogging the telephone line. 
Hope you get what I meant.
How come telephone line can be use DSL connection is active?
All DSL come with a mirco-filter. Is somehow act like a spilter,
or traffic warden of a Expressway. Every wire get it own specific
line. Is line, the line have to be plug into line, morden plug into
morden and phone plug into phone. Somehow is similar to how traffic
on the Woodland Causeway. The bus and lorry one lane, the car in
this lane, while the motorcycles in the another lane. And
of-course, is somehow not very stable.
Hops this helps.