Headset on telephone hack

Some time ago, I had an idea: put a headset on a telephone.

Unfortunately I haven't electrical knowledge or experience. However I think the procedure should be very simple.

I disassemble telephone and headset cables. Noted that speakers cables are green/red, so probably are some standard. I crossed them to take headset speaker on telephone.

But when the speaker receive some signal, a noise invades. For my happiness the noise have correlation with the original signal ( eg. voice trough the line). So I believe that even though follow the same standard have different input/output specifications,
something like tubes, which despite serving for the same thing, does so with different capabilities.

Despite microphone don't have equal cable colours, I tried and worked well. 😛😎

If you want just reply, my questions are:

• What the unit measure of an speaker, and how to discovery it?
• I have to buy a speaker with the same specifications of the original, ok?

Somebody can help me?

obs:
sorry by my confused english, I'm brazilian.
​