Sound problem

I have a problem with my Asterisk implementation.
I have 20 SPA941 VoIP phones with two lines each. One is the interior and the other the direct line. On Asterisk i have an TE120P with a E1 line.
The problem is on the direct line of the phones. When i call from one phone’s direct line all is ok (i can hear the called party and he can hear me). The problem is where someone call me on the direct line (i can hear him but he can’t hear me).
I have one phone that is working and the caller can hear me.
All the phones have the same firmware and settings, and the settings in asterisk are the same for all extensions.
I tried all the codec combinationsbut it’s not working.
If someone expirienced this problem PLEASE help me. I don’t have any ideea any more.
Thank you