Budge Tone 100 loging of

Hi I have a new Hi My Budge Tone 100 connected to my Asterisk. 3to 4 times during 24 hour it's log of the Asterisk and the NET led flash. If I lift handset and try to make a call, I get an error tone, and the telephone logs on again.
I have a Cisco 7960 SIP protcol, and it works great.

Anyone have the same problem with SIP phones?
Please if you know why or have a solution on this, give me a mail.
My Asterisk is a home version 2.1
Ulf