Multiple office locations

We are offering phone support to another established office in another town, we run separate businesses. When THEY make calls from their phone, caller ID (what the client see) should be THEIR phone number (not ours). It also would be nice to have an identification ring for when their calls get transferred here, we know to answer it differently. We have set a specific ring type set (in our phone's contacts), but that only works when that office calls us directly. We have call display set to "number called" so that is a temporary fix, but we are missing the caller display.