On an OBi202 the PHONE1 jack is wired to work with a 2-line phone. Its two centre contacts (3 and 4) are wired for ph1 and contacts 2 and 5 are wired for ph2. Those contacts 2,5 in the PHONE 1 jack are hard wired to contacts 3,4 in the PHONE2 jack. There is no way to remove that within the 202.
You need to use an external means to stop the 2,5 contacts in the PHONE1 jack connecting to the 2,5 contacts in the socket on the Panasonic phone. Simplest way is to replace the cable with one that only contains wires for 3,4. Or some types of 'line splitter' will do what you need.