Here is the trace log for both the IPO and the D100. I replaced the public IP's and Mac's with the description for security purposes.
IP Office Trace
10:54:51 68466794mS SIP Rx: UDP CustPublicIP:5056 -> TelAgilityIPO:5056
REGISTER sip:TelAgilityIPO:5056 SIP/2.0
From: <sip:6081@TelAgilityIPO>;tag=f69b50-8c8120a-13c0-6006-10a9d-0-10a9d
To: <sip:6081@TelAgilityIPO>
Call-ID: f8f42e-8c8120a-13c0-6006-10a9d-0-10a9d
CSeq: 1 REGISTER
Max-Forwards: 70
Supported: replaces,100rel,timer
User-Agent: Avaya D100 (D100_1.2.7_0.9.6 D100MACaddress)
Content-Length: 0
Via: SIP/2.0/UDP CustPublicIP:5056;branch=z9hG4bK-10a9d-4117628-0-f4f7c8
Contact: <sip:6081@CustPublicIP:5056>
Expires: 180
10:54:51 68466794mS Sip: SIPDialog f704f7d0 created, dialogs 15 txn_keys 1 video 1 presentation 1 camera 1 unsupp audio 0
10:54:51 68466794mS SIP Tx: UDP TelAgilityIPO:5056 -> CustPublicIP:5056
SIP/2.0 401 Unauthorized
Via: SIP/2.0/UDP CustPublicIP:5056;branch=z9hG4bK-10a9d-4117628-0-f4f7c8
From: <sip:6081@TelAgilityIPO>;tag=f69b50-8c8120a-13c0-6006-10a9d-0-10a9d
Call-ID: f8f42e-8c8120a-13c0-6006-10a9d-0-10a9d
CSeq: 1 REGISTER
User-Agent: IP Office 11.0.0.2.0 build 23
Allow: INVITE,ACK,CANCEL,OPTIONS,BYE,REFER,NOTIFY,INFO,SUBSCRIBE,REGISTER,PUBLISH
Digest nonce="13b0a14be319844302f4",realm="ipoffice",algorithm=MD5
Server: IP Office 11.0.0.2.0 build 23
To: <sip:6081@TelAgilityIPO>;tag=1d96fbf2f9c7d24f
Content-Length: 0
10:54:51 68466887mS SIP Rx: UDP CustPublicIP:5056 -> TelAgilityIPO:5056
REGISTER sip:TelAgilityIPO:5056 SIP/2.0
From: <sip:6081@TelAgilityIPO>;tag=f69b50-8c8120a-13c0-6006-10a9d-0-10a9d
To: <sip:6081@TelAgilityIPO>
Call-ID: f8f42e-8c8120a-13c0-6006-10a9d-0-10a9d
CSeq: 2 REGISTER
Max-Forwards: 70
Supported: replaces,100rel,timer
User-Agent: Avaya D100 (D100_1.2.7_0.9.6 D100MACaddress)
Authorization: Digest username="6081",realm="ipoffice",nonce="13b0a14be319844302f4",uri="sip:TelAgilityIPO:5056",response="10ce454886ee9931ddc76658939e7a7b",algorithm=MD5
Content-Length: 0
Via: SIP/2.0/UDP CustPublicIP:5056;branch=z9hG4bK-10a9d-4117678-0-f4f920
Contact: <sip:6081@CustPublicIP:5056>
Expires: 180
10:54:51 68466887mS Sip: SIP REG: ProcessRegistrationRequest, delete phone receiver 578 if phone size is 0 (size: 0)
10:54:51 68466887mS Sip: SIP REG: extn 6081 address restricted
10:54:51 68466888mS SIP Tx: UDP TelAgilityIPO:5056 -> CustPublicIP:5056
SIP/2.0 503 Service Unavailable
Via: SIP/2.0/UDP CustPublicIP:5056;branch=z9hG4bK-10a9d-4117678-0-f4f920
From: <sip:6081@TelAgilityIPO>;tag=f69b50-8c8120a-13c0-6006-10a9d-0-10a9d
Call-ID: f8f42e-8c8120a-13c0-6006-10a9d-0-10a9d
CSeq: 2 REGISTER
User-Agent: IP Office 11.0.0.2.0 build 23
Allow: INVITE,ACK,CANCEL,OPTIONS,BYE,REFER,NOTIFY,INFO,SUBSCRIBE,REGISTER,PUBLISH
Retry-After: 180
Server: IP Office 11.0.0.2.0 build 23
To: <sip:6081@TelAgilityIPO>;tag=9cb768500d9a8f67
Content-Length: 0
D100 Trace
[2000/01/01 18:57:26.930] 5038 Hsif_menuLoggingInState
[2000/01/01 18:57:26.930] 5039 1st Login
[2000/01/01 18:57:26.933] 5040 [/D100.cfg] reading.
[2000/01/01 18:57:27.196] 5041 - finished.
[2000/01/01 18:57:27.196] 5042 [/D100prov.txt] reading.
[2000/01/01 18:57:27.391] 5043 - OK. There is no difference between Provisioning File and current settings.
[2000/01/01 18:57:27.391] 5044 [/D100settings.txt] reading.
[2000/01/01 18:57:27.593] 5045 - finished.
[2000/01/01 18:57:27.642] 5046 [D100_Italian.lng] reading.
[2000/01/01 18:57:28.401] 5047 - finished.
[2000/01/01 18:57:28.401] 5048 [D100_German.lng] reading.
[2000/01/01 18:57:29.157] 5049 - finished.
[2000/01/01 18:57:29.158] 5050 [D100_Spanish.lng] reading.
[2000/01/01 18:57:29.913] 5051 - finished.
[2000/01/01 18:57:29.913] 5052 [D100_French.lng] reading.
[2000/01/01 18:57:30.675] 5053 - finished.
[2000/01/01 18:57:31.538] 5054 [system/huntgroup/scn_huntgroup_list] reading.
[2000/01/01 18:57:31.602] 5055 [E] [/system/huntgroup/scn_huntgroup_list] File Not Found.
[2000/01/01 18:57:31.602] 5056 [E] HTTP Client Return Error:-1
[2000/01/01 18:57:31.605] Login Common Files use previous setting. [download error - scn_huntgroup_list]
[2000/01/01 18:57:32.718] 5057 ### Login request [6081] - HsIf_CallHdl=0x4b1204
[2000/01/01 18:57:32.719] 5058 [sipusers/D160_6081@TelAgilityIPO.cfg] reading.
[2000/01/01 18:57:32.890] 5059 - finished.
[2000/01/01 18:57:32.891] 5060 [D160_6081@TelAgilityIPO.keys] reading.
[2000/01/01 18:57:33.090] 5061 - finished.
[2000/01/01 18:57:33.388] 5062 HS1 EVENT_LOGIN_DENIDED
[2000/01/01 18:57:33.391] 5063 [E] Sipap_MtfRegClientStateChangedCB(term=6081) - failed to send REGISTER(reason=Response server failure received) call Hsif_LoginDenided
MTF_PRT 01/01/00 18:57:33 IPP_SIPCONT : WARN - handleRegStateFailed() msg=503 name=6081
[2000/01/01 18:57:33.413] 5064 [E] Sipap_MtfRegClientStateChangedCB(term=6081) - State Terminated(reason=User request)
[2000/01/01 18:57:33.422] 5065 ### Logout completed [6081]