Found this at Polycoms website, while not Obi specific it gives a more laymanish explanation of the nuances of digit maps.
http://support.polycom.com/global/documents/support/technical/products/voice/Understanding_Digit_Maps_Tech_Tip.pdf
Below is a link to the full blown geek type information on digit maps, from the specifications group that help define the standards. Be forewarned that some of this might not apply to Obi boxes:
http://tools.ietf.org/html/rfc3435#section%E2%80%902.1.5
Good info. Is there a link to a "Gold reference" for the Obi?
See the Device Admin Guide
http://www.obihai.com/docs-downloads