Documentation is limited and the setup was complicated by the differences in terminology between the VOIP provider (Gradwell) and the phone setup screens.
Having spent most of a day setting the phones up we are living with the issue that, if they are rebooted, the phone automatically diverts all calls to voicemail. There doesn't seem to be an answer to this. The VOIP provider has said that this is a problem with the phone setup rather than their system. Making a call resolves the problem and so we're living with that until we have the energy and time tor resolve it properly.
The build quality is cheap, as you would expect for a budget phone, and the documentation is basic. The sound quality is very good and other than the voicemail issue, it appears to be working well.