Times Ringing before Mailbox takes call

Hi Community,

I have the following problem with my mailbox. I set up an freePBX System behind an ISDN Telephone System. Everything works fine so far. The Problem I have is that, when activating the Mailbox, and no VOIP Telephone is registered, it immediately takes the call. I would like the System to wait some time, so that I can pick up the phone with my ISDN Telephone. When a VOIP Client is registered it works fine. I can take the call with my ISDN Telephone or the VOIP Telephone.

Are there any solutions for this Problem?

Best Regards