[Serusers] Weird register issue

Weird issue, only with registers. Sometimes when I start ser (and openser, same issue with both) my test phone (behind nat) will not register allways getting 401 unauthorized back. But wait a minute and it will register (without resetting ser or the phone). Is this a built in feature or a bug (it only happens when I reset ser/openser after 60 seconds the server and phone behave normally). Got me stumped as the configs on the phone and ser don't change. REGISTER sip:XX.XX.XX.XX SIP/2.0. Via: SIP/2.0/UDP 192.168.10.25:5060. From: <sip:2959770500@XX.XX.XX.XX;user=phone>;tag=1886287424. To: <sip:2959770500@XX.XX.XX.XX;user=phone>. Call-ID: 147690828(a)192.168.10.25. CSeq: 1 REGISTER. Contact: Nathan Gross <sip:2959770500@192.168.10.25:5060;user=phone;transport=udp>;expires=60. User-Agent: Cisco-CP7905/1.02-040406A. Allow: ACK, BYE, CANCEL, INVITE, NOTIFY, OPTIONS, REFER, REGISTER. Content-Length: 0. SIP/2.0 401 Unauthorized. Via: SIP/2.0/UDP 192.168.10.25:5060;rport=62686;received=YY.YY.YY.YY. From: <sip:2959770500@XX.XX.XX.XX;user=phone>;tag=1886287424. To: <sip:2959770500@XX.XX.XX.XX;user=phone>;tag=264d52ef5b2f408fc315ea210b15c82e .5681. Call-ID: 147690828(a)192.168.10.25. CSeq: 1 REGISTER. WWW-Authenticate: Digest realm="XX.XX.XX.XX", nonce="42b9e43447292eaaa08c9b86dcc1e23344ac3c6a", qop="auth". Content-Length: 0. Thank You Anthony Dean Network Design & Support Tristar Communications This correspondence is considered confidential and any reproduction for the purpose of public disclosure is forbidden without written permission by the author signed above. If you are not the intended recipient, please immediately notify the sender and delete any copies. If you are responding to this e-mail, please append or attach all previous correspondence as a point of reference.