What does it mean if only one of the phones in the house rang, and it was engaged when it was picked up?

Question

In the house there are multiple phones connected to the same number, so when that number is called, all phones ring and you can answer whichever one you're closest to. But today only one rang, and when it was picked up, it was engaged, yet later like normal they all rang when someone else called

Answer 1

The question is not detailed but it seems to me that you have two or more phones connected in parallel.

Only one is ringing only means that the line current is not enough to ring all of the units. Check this link.

http://en.wikipedia.org/wiki/Ringer_equi...