Why would 1 phone jack make the others not work?

I have 3 phone jacks in my house. 1 which is a double jack, which I wired in myself. After the wires leave the jackbox outside my house, i put a splitter in. when i test every jack, my tester claims its all wired correctly. but when i hook my fax machine up to the newly installed double jack, it makes the other 2 jacks act like a NO LINE. and the second double jack which my DSL modem is connected, dissconnect when the fax machine is in use.