fanvil x4g working then sip register fail

[frog]

a new install of 3cx latest using fanvil x4g phones. All phones are setup as hotdesk. a number of them which were working have started to display SIP REGISTER FAIL. Although I can connect to them via the web interface they do not let me log using the hotdesk web ui password. The only way to get them back is to do a factory reset.

Any ideas?