My last house had all my phone lines connected together in one place. I thought, perfect, I’ll put a gigabit switch here! I removed the ethernet wires, put ethernet heads on them, then went throughout my whole house doing the same at every phone jack. I got an ethernet cable tester. Not a single one worked. I could get the occational partial connections, but no full connection. I made a few ethernet cables just to make sure I wan’t doing it wrong, and they worked fine.

There may be another explaination as to what I saw, but I tried for several days trying to get ethernet throughout the house to no avail.