Problem with busy internal

Good morning,
We have the following problem:
When an extension is engaged in a call is not busy, so if someone calls him while engaged in another conversation, the call enters and stays in the background rather than giving busy signal. How can I configure the file extensions to avoid this anomaly? Or is it a different problem?
Thank you

You would need to provide console output and further description of your problem exactly. The technology in use (are you using SIP) would also be useful.