Config working with sip server

I config asterisk to receive sip call from other sip server.
two sip user 3000 and 3001 register with asterisk and could call each other.
asterisk register with the sip server as a client.when I call a sip extension of asterisk(example 3001),asterisk reply 407 Proxy Authentication required .sip proxy send new invite with authorization ,then asterisk reply with 488 response.
can any one help me?
thanks very much.

Try using only ulaw and see if you still have the problem.

I have the same problem,my configure is as following:

port = 5060 ; Port to bind to (SIP is 5060)
bindaddr = ; Address to bind to (all addresses on machine)
context = from-sip-external ; Send unknown SIP callers to this context
callerid = Unknown
;externip =
;localnet =
;localnet =
;#include sip_nat.conf
#include sip_custom.conf
#include sip_additional.conf
#include additional_a2billing_sip.conf