Unseen problem

Sorry for the title but this problem is really driving me crazy. I have a Trixbox CE 2.2 setup with 9 phones all Linksys 941,922,942, PAP2T. I have a branch connected using IPSEC VPN utilizing two Linksys RVL200 (Firmware 1.1.7). The VPN working fine I am running Remote Dekstop off it. However, the linksys phone (922) that I connected there works only for one day. The next day it will show Failed Registering!! It works only if I change the IP.
I tried to debug the registeration process from Trixbox using "sip debug ip..".
It shows that there are requests comming from the phone but without the authentication info, the following is the debug output:

<-- SIP read from 192.168.1.245:5060:
REGISTER sip:192.168.0.9 SIP/2.0
Via: SIP/2.0/UDP 192.168.1.245:5060;branch=z9hG4bK-6a111a2b
From: "Zeina" ;tag=2e22310ef74308fao0
To: "Zeina"
Call-ID: 8dabaa08-7a1a5198@192.168.1.245
CSeq: 20336 REGISTER
Max-Forwards: 70
Contact: "Zeina" ;expires=3600
User-Agent: Linksys/SPA922-4.1.18
Content-Length: 0
Allow: ACK, BYE, CANCEL, INFO, INVITE, NOTIFY, OPTIONS, REFER

--- (11 headers 0 lines) ---
Using latest REGISTER request as basis request
Sending to 192.168.1.245 : 5060 (non-NAT)
Transmitting (no NAT) to 192.168.1.245:5060:
SIP/2.0 100 Trying
Via: SIP/2.0/UDP 192.168.1.245:5060;branch=z9hG4bK-6a111a2b;received=192.168.1.245
From: "Zeina" ;tag=2e22310ef74308fao0
To: "Zeina"
Call-ID: 8dabaa08-7a1a5198@192.168.1.245
CSeq: 20336 REGISTER
User-Agent: Asterisk PBX
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, SUBSCRIBE, NOTIFY
Contact:
Content-Length: 0

---
Transmitting (no NAT) to 192.168.1.245:5060:
SIP/2.0 401 Unauthorized
Via: SIP/2.0/UDP 192.168.1.245:5060;branch=z9hG4bK-6a111a2b;received=192.168.1.245
From: "Zeina" ;tag=2e22310ef74308fao0
To: "Zeina" ;tag=as74e470fc
Call-ID: 8dabaa08-7a1a5198@192.168.1.245
CSeq: 20336 REGISTER
User-Agent: Asterisk PBX
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, SUBSCRIBE, NOTIFY
WWW-Authenticate: Digest algorithm=MD5, realm="alarbashpc", nonce="05749749"
Content-Length: 0

When debuging other phones there will appear same sequence but afterward there will be exchange of authentication info. With this setup this sequence just repeats.

Any one can hint were is the problem?