Conference Call : Can't hear the voice until a keypress

Hi everyone,

We’re using Asterisk and it’s really amazing. But recently, users have reported that when they call a conference bridge, they didn’t hear the voice of the “robot” which says the welcome message and asks to enter the digits. On the phone, we can see that the call is established but we can hear nothing. We have to press a key and then we’ll heard something.

Do you have any clues? Any ideas?

Thank you a lot for your help,