Phones periodically report 'NR' or 'No Service' / All my phones have stopped working

Technical FAQs
#1

This is generally caused by one of:

  1. The DHCP server has stopped functioning.

  2. The DHCP server has handed out a duplicate IP address (often caused if an ASDL router is serving DHCP, rather than a server)

  3. The DNS server is no longer serving the name of the PABX correctly.

  4. There is a LAN issue, perhaps a faulty switch or router.

#2