After the extension hang up the phone, is not able to answer at the second call

Hi all,

my scenario: freepbx distro with ISDN card.

The problem:

  1. On external call, all the extensions ringing. (correct)
  2. one extension (100) answer the call (correct)
  3. freepbx get another external call , all other extensions ring but the extension 100 not indicate the call. If the call end, the extension 100 is not able to answer the call 2 (In the meantime it is still ringing on all other phone extensions). (not correct)

After the ext 100 hang up the phone, is not able to answer at the second call that In the meantime it is still ringing on all other phone extensions.

How can I resolve this issue?

Thank you.
Andrea