Splitting CatV cable

My non-profit is expanding to a building that is physically separated from our current office, which contains the DSL and phone connections. Our landlord ran CatV cable to every room in the new office building – he wants to split the outlet in each office to work both data and phone. My question is: how will this effect the DSL speed and respone? My understanding is that splitting the cabel 6 pairs for data and two pairs for phone will slow the internet down. Is this true?