407 Proxy registration required

I am using asterisk 1.4 and doing basic configuration, I am able to register 3 xlite phone from my asterisk server and able to make call between them.

Now i am trying to configure it for outbound/inbound call , asterisk register with gafachi(my sip provider) successffully but when i make call to gafachi it say me 407 proxy registration required and cancel the call.

Any idia about that i am sending my wireshark catured file with this .
60 8.261741 SIP Request: REGISTER sip:a2541shRzJKLjtQA.sip.gafachi.com
64 8.744117 SIP Status: 200 OK (1 bindings)
255 45.030046 SIP/SDP Request: INVITE sip:16462781042@, with session description
256 45.032479 SIP Status: 407 Proxy Authentication Required
260 45.533438 SIP Request: ACK sip:16462781042@

Thanks in advance
Yogesh Chaturvedi