Busy signal

We have 4 trunks pots verizon lines in new york connected to an asterisk box using a Digium AEX804 with a quad ports.

We can emit and receive calls but sometime when we try to make a call we get a “busy-signal”.

Looking at the asterisk log, it seems that asterisk takes a free line but close it a few seconds after without any

rings before the busy-signal.

Does somebody can help us?

Thanks in advance