503, and reinvites

Hello all,

i am looking for some help or suggestions for the following topics:
1)
i have two SIP peers registered to the PBX.
after Peer1 INVITE Peer2, often, expecially if it is the first message issued after the peers have been registered, the PBX reply with 503 Server Error.

After Peer1 INVITE Peer2, and Peer2 has already replied 200OK to the pbx, pbx issue another INVITE to Peer2,
See the log below:

(PBX = 192.168.0.20, peers are .27 and .28)
FROM                 TO
192.168.0.27          192.168.0.20          SIP      Request: REGISTER sip:192.168.0.20 
192.168.0.20          192.168.0.27          SIP      Status: 100 Trying    (1 bindings) 
192.168.0.20          192.168.0.27          SIP      Status: 200 OK    (1 bindings)  
192.168.0.28          192.168.0.20          SIP      Request: REGISTER sip:192.168.0.20  
192.168.0.20          192.168.0.28          SIP      Status: 100 Trying    (1 bindings)  
192.168.0.20          192.168.0.28          SIP      Status: 200 OK    (1 bindings) 
192.168.0.27          192.168.0.20          SIP/SDP  Request: INVITE sip:10028@192.168.0.20, with session description
192.168.0.20          192.168.0.27          SIP      Status: 407 Proxy Authentication Required 
192.168.0.27          192.168.0.20          SIP      Request: ACK sip:10028@192.168.0.20  
192.168.0.27          192.168.0.20          SIP/SDP  Request: INVITE sip:10028@192.168.0.20, with session description 
192.168.0.20          192.168.0.27          SIP      Status: 100 Trying  
192.168.0.20          192.168.0.28          SIP/SDP  Request: INVITE sip:ann28@192.168.0.28:5060, with session description 
192.168.0.28          192.168.0.20          SIP      Status: 180 ringing  
192.168.0.20          192.168.0.27          SIP      Status: 180 Ringing 
192.168.0.28          192.168.0.20          SIP/SDP  Status: 200 OK, with session description    
192.168.0.20          192.168.0.28          SIP      Request: ACK sip:ann28@192.168.0.28:5060   
192.168.0.20          192.168.0.27          SIP/SDP  Status: 200 OK, with session description 
192.168.0.20          192.168.0.28          SIP/SDP  Request: INVITE sip:ann28@192.168.0.28:5060, with session description 
192.168.0.27          192.168.0.20          SIP      Request: ACK sip:10028@192.168.0.20
192.168.0.20          192.168.0.27          SIP/SDP  Request: INVITE sip:ann27@192.168.0.27:5060, with session description 
192.168.0.27          192.168.0.20          SIP/SDP  Status: 200 OK, with session description
192.168.0.20          192.168.0.27          SIP      Request: ACK sip:ann27@192.168.0.27:5060 
192.168.0.28          192.168.0.20          SIP/SDP  Status: 200 OK, with session description                        644
192.168.0.20          192.168.0.28          SIP      Request: ACK sip:ann28@192.168.0.28:5060                        417

So i don’t understand why the pbx reinvite the peer 192.168.0.28.

Any help is really appreciated.
Many thanks