My iPhone (Classic) was failing to get DHCP via one of the WiFi networks in the office. Some convoluted Googling later, I found that perhaps the clash is due to the Billion DHCP server implementation not liking the AirPort Extreme IPv6 Tunneling option being enabled.
Change the IPv6 configuration in your AirPort Extreme to:
And the basestation reboots by itself, which left me with working DHCP.
Yes, I have the WiFi basestation(s) as bridges onto the DHCP-serving router.