This is incomplete. The normal flow is-> REGISTER<- 401-> REGISTER<- 200 or 403
The digest includes the domain, if the user and password match perhaps the problem lies there...
Seeing the firmware version in the User Agent header this phone is probably build on the same platform as the 79XX series phones and having the same sip-stack.
From experience I can tell you that sip-stack is really really horrible. I don’t know if you have bought a lot of these phone or just testing one.