[Asterisk-Users] fail to make call

[unplug]

Hi
  I have the following configuration

         |
UA1 --|------ asterisk1 -----------------------+
UA2 --|------ asterisk2 -----------------------+ DB
UA3 --|------ asterisk3 -----------------------+
UA4 --|------ asterisk4 -----------------------+
         |

All UA is located in the same area.  A seperated PC is used as a
centralized DB for storing a common dial plan, user account and
register infomration.
UA1 can make call to UA2,UA3 and UA4.
UA2 can make call to UA1, UA3 but not UA4.
UA3 can make call to UA4 but not UA1, UA2
UA4 failed to make call to all UA.

>From the CLI and log below, asterisk shows it can't create channel.
As I expect, all UA should able to find each other.  However, some of
them are failed to find others.  I have no idea why they can't find
each other well.  Is it the configuration problem?  Anyone can help?

    -- Executing Dial("SIP/871966629896-5373", "SIP/871966760539|15")
Jun 20 17:42:16 NOTICE[23355]: app_dial.c:1040 dial_exec_full: Unable
to create channel of type 'SIP' (cause 3 - No route to destination)
  == Everyone is busy/congested at this time (1:0/0/1)
    -- Executing Hangup("SIP/871966629896-5373", "")

NOTICE[28269] app_dial.c: Unable to create channel of type 'SIP'
(cause 3 - No route to destination)