SIP/2.0 404 Not found (unknown domain)

I try to register a MediaPack MP-112 with Asterisk. The registration is SIP, but i have the folowing when i try a registration:

<— Transmitting (no NAT) to 172.22.8.218:5060 —>
SIP/2.0 404 Not found (unknown domain)
Via: SIP/2.0/UDP 172.22.8.218;branch=z9hG4bKac2078622765;received=172.22.8.218
From: sip:9999@172.22.8.218;tag=1c2078617430
To: sip:9999@172.22.8.218;tag=as59d2c9d2
Call-ID: 140959068211200017525@172.22.8.218
CSeq: 197 REGISTER
User-Agent: Asterisk PBX
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, SUBSCRIBE, NOTIFY
Supported: replaces
Content-Length: 0

<------------>
[2008-01-25 11:28:21] NOTICE[28035]: chan_sip.c:14758 handle_request_register: R
egistration from ‘sip:9999@172.22.8.218’ failed for ‘172.22.8.218’ - Not a loc
al domain
Scheduling destruction of SIP dialog ‘140959068211200017525@172.22.8.218’ in 320
00 ms (Method: REGISTER)

I don’t know what happend, the IP of the MP is 172.22.8.218, however i can call from Asterisk to the MP very easy, but I want to register the MP to Asterisk like a trunk SIP.
This is not the first time i have this problem with a SIP trunk, I have this problem with two Asterisk boxes too before and I change the trunk to IAX and work fine, but the MP don’t have IAX. I need help, please.