I personally think the wire in CAT5 cable and the wire used in phone lines is a bit too thin to work reliably in many breadboards.
On the breadboards I use (all of them form DX), I get a tight fit with CAT5 cables. Again, the fact that they are color coded is a huge advantage. The only breadboard that isn't ok is a very old, Brazilian-made breadboard I've had for 20 years. I've abused it so much with thick pin inductors, potentiometers, HV diodes and the like, that most of its holes are expanded. Even commercial jumper cables will fit loosely on it.