When I worked for a company taking calls to help people set up their DSL for the first time we'd often start diagnosing by reading through the instructions. Instruction #1, don't use a phone line splitter, plug directly into the wall.
This call would happen about once a week:
Me: "Sir, can you describe the equipment starting at the wall jack and working your way to the modem?"
Customer: "Yeah, there's a splitter plugged into the jack, and the phone cord plugged into that, which is plugged into the modem."
Me: "Sir the splitter is going to prevent your internet from working, can you remove it?"
Customer: "Sure...", *clunk* *click* *dialtone*
Can you guess what was plugged into the other side of that splitter?