I have a client that has a TDA-50 system and they have 2 doorphones hooked up to the system. They have been hooked up and working for about 4-5 months and now, out of the blue, they aren't working. Here's the issue: When you press the button, the doorphone calls into the house on all extensions. When you pick up one of the phones, you can hear the person at the DP, but they cannot hear you. I have tried replacing the DP card, bringing a DP to the system directly, but the problem is still there. Any ideas?

You did not say if you replaced or tried a new doophone?