У параметра qualify есть на вскользь я помню время ожидания отклика. Увеличьте его
The value in qualify = represents the timeout after a packet is sent before we consider the peer to be unreachable. If the packet is not responded within 1 second, asterisk will keep trying until 7 packets have failed. At this point, asterisk won't try again until the next 60 cycle period completes.