why can i hear my own voice through the earpiece when speaking?

my cordless phone has a base unit with handset + another handset.

when i speak into the handset i can hear my own voice through the earpiece as well as the caller.

I disconnected the telephone line and tried just the handsets (both of them, seperatly) to see if it was the same. it was and both handsets hand the same problem? so its not a handset? is it the base unit?

I have got wireless broadband stationed near where the phone base unit normally sits. But to test them on their own i moved them into the next room?

Make- Uniden, Model- WDECT 2355/2355 + 1 Phone

Thanks