By default, all incoming calls ring the phone(s) connected to the OBi's Phone Port. Therefore, incoming calls from a landline connected to the OBi's Line Port will automatically ring its phone. All you should need to do is connect your landline to the OBi's Line Port.
I assume you already have PrimaryLine on your OBi set to the SPx Port that is Callcentric (PrimaryLine only affects outbound calling, not inbound calls). Should you wish to place a call via the lanline instead, you would simply preface the number with **8.
If you're on either the landline or Callcentric and you receive an incoming call on the other, you'll receive a Call-Waiting indication and can answer the other call using the hook-flash.
You'll probably end up wanting all your phones connected to the OBi and none connected directly to the landline.