When configuring a provisioning link (e.g. for 3CX) in DHCP server on a Draytek firewall, and are using certain phones (particular Snom), you may run into the issue where the firewall is complaining about illegal characters when attempting to add the link through the web interface.

The solution is to configure the option through the CLI. Log in using either telnet or SSH (whichever is enabled), and type:

srv dhcp option -e 1 -i 1 -c 66 -v http://192.168.1.10:5000/provisioning/ajehdukzidueh/cfg{mac}

Take particular care to the -i option, which defines the LAN subnet to be used: in this case, 1 references LAN 1 on the Draytek.