Is there something that needs to be done to allow an IP phone to work Both offsite as well as onsite with out having to change those settings in the DB?
Unfortunately with a Minet phone, no, it just doesn't work that way without a Mitel Border Gateway.
If you have an Intertel IP phone working remotely, it has an Auto option and it works quite well in most cases, but Minet phones do not support this and it must be manually switched for whether are on the network Natively or via NAT.
The only other way I know around this would be to put a public IP address directly on the Mitel, but that can have some pretty serious security implications in itself.