I have an Electra Professional Level II system and using ETW-16DC-1 telephone. when I hit the FNC key and press Line 1 on the phone I get the message Vacant.I do the same with Line 2 and then it displays CO Line 01. How do you program Line 1 to be CO line 01 and Line 2 to be CO Line 02? This issue is only happening on one phone.