Are you having overlapping numbers or is it just that you want to keep the internal numbers same, while using different external DIDs ? If its the latter, its as easy as configuring Translation patterns.
You can have the telco send a DID 2840 while translate that to an internal extension 1870.
You create a trans. pattern and the pattern will be for 2840. The voice gateway (either H323 or MGCP) will send calls for 2840 to the Callmanager. Callmanager looks up this number matches 2840 and translates that to 1870. Where you specify 1870 is in the Called Party Transformation mask field under translation pattern.
The IP Phone can be physically assigned the extension 1870.
UC Solutions Architect
Pacific Northwest | CDW
CCIE Collaboration #17135 Emeritus