We are in the process of converting our offices to SIP and have run into a situation where off premise call forwarding is failing. It seems that the calling number becomes the number of the caller and the called number becomes the off premise number the phone is forwarded to. Well, our SIP provider does not know the callers number as a vailid number and rejects the call. Is there a way to keep the calling number as the extension/phone number of the forwarded phone?