The signal that rings your phone is generated by your Obi. The cadence is programmable and the Obi contains several preset ones.
If you are using two Service Providers you can assign a different cadence to each. However you only mention Callcentric so I assume you have one SP. AFAIK it is not possible to have the Obi use a different ring cadence depending on the caller's number.
However the Obi will pass the CallerID to the phone and some phones can match incoming numbers to ones in their phonebook which have been assigned distinctive rings.
