That is the way our older installed DSL is; there is one 'filter' right after telco. line enters the house which separates the telephones from the DSL. After that it is separate wiring to the phones around the house. In newer and 'self' installations there is often a filter at each phone which prevents DSL from getting into the phones and/or they from interfering with the DSL. Don't think the OP has enough circuit analytical skills to trouble shoot the situation. And seems to be averse to totally disconnecting the telephone pair outside to prove whether it is IN or or OUTSIDE his house? It may be one side open outside, something shorting (or semi shorting) the line, defective modem, defective filter. Agree that in most cases if it's outside it's telco responsibility. If inside, these days of competition etc. customer responsibility. But he should be careful about not messing it up more for the telephone tech to have to fix!