ATA De-registers

[Tacitus]

My ATA - Grandstream 801v2 - has recently started to de-register itself.  A paperclip reset makes it re-register, but I really need to find a cause.

The obvious could be a faulty ATA, but putting that aside shouldn't the ATA re-register automatically in the event of a power cut or network interuption.

Anyone any ideas as to a likely cause for de-registration?