If your toll free 800 number is from your VOIP service, you do not have to do anything. Once Voicent Gateway is setup with your VOIP service, any call to your 800 number should be automatically forwarded to Flex PBX.
If your toll free number is from a different company, you can normally set up calls to your 800 number to be forwarded to a different phone number. That number can be any phone number. If you use regular analog phone lines for Flex PBX, forward the 800 calls to the analog lines. If you use VOIP for Flex PBX, you can purchase an incoming DID number, which is basically a regular phone number that can be called from any telephone. Once the DID number is set, forward your 800 number to the DID number.