asterisk users - Aug 2006 - Declined to talk, Call rejected: 603 Declined

I have the following configuration in using of 2 asterisks.


ARA(DB) / AST1 (10.0.1.0)-----------UA1 (ext:3634, IP:192.168.1.10)
              \ AST2 (10.0.1.5)-----------UA2 (ext:4634, IP:192.168.1.20)

I have a dial plan in order to allow UA1 make calls to UA2.

exten => 4634,1, Dial(SIP/4634@10.0.1.5)

UA1 showed "Declined to talk, Call rejected: 603 Declined" when UA1
tried to make call to UA2 (execute the above dial plan).

I have already made UA1 and UA2 as peer for each other but error 603
is still showing.

What is the meaning of error 603?  How can I make the above situation
work for UA1 and UA2 to make call to each other?  Anyone can help me?