[Serusers] Registering with sipphone

Hello, I have a problem about registration and online/offline status. 1. I open the SIPphone first, it log in and i can make a call (my status online) several times. One hour later i check it but my status is still online and it's never be offline. 2. Although I do nothing about configuration, several hours later, when i try to make a call SIPphone can't log in again . So i check SIPphone Diagnostics(shown below). any idea, what could be the problem.? RECEIVE TIME: 9275247 RECEIVE << 192.168.3.151:5060 SIP/2.0 401 Unauthorized Via: SIP/2.0/UDP 192.168.3.152:5060;rport=5060;branch=z9hG4bKFEF502CD31E84C4E94F4CDC0D0FDD367 From: 8877 <sip:8877@test.com>;tag=4270805189 To: 8877 <sip:8877@test.com>;tag=b27e1a1d33761e85846fc98f5f3a7e58.d43e Call-ID: BD0C1974B3A84098AA64B66B7EA218AF(a)test.com CSeq: 29697 REGISTER WWW-Authenticate: Digest realm="test.com", nonce="42539e90509011eb128ee451b0e8675d36c52a20" Server: Sip EXpress router (0.8.11 (i386/linux)) Content-Length: 0 Warning: 392 192.168.3.151:5060 "Noisy feedback tells: pid=4024 req_src_ip=192.168.3.152 req_src_port=5060 in_uri=sip:test.com out_uri=sip:test.com via_cnt==1"