is there anyone who can tell me the reason why recordings that shows up in voicemail application are empty?

The situation is as follows:

I have na number 800 that has a voicemail activated via button (simple on/off redirection to **800, where ** is the number of voicemail). After calling, there is a script sayng "leave the message" and the beep. After finishing the call, the recording shows in the voiemail app, but it's lenght is 0sec, also the downloaded file is empty too. What am I missing?