Andrew, will you please complete your profile for us. :read: Thank you, MrG.
It is a limitation of the system. You can have the overflow for the hunt group but you can't forward the overflow ext.
There may be a work around if you have an open CO line.
Have the overflow ext. an XDP. Loop it back into an unused CO and set up that unused CO as a DIL 1-1 to an Ext. that has all calls forwarded to the outside #.
Make sure you put that unused CO in it's own trunk group with no outgoing assigned so that the normal sets can't grab it when calling out.
lso you didn't mention if you have a TVS or any kind of voicemail.
MrG