The phone only works with the RCA base unit (25414RE3 or 25415RE3) and both phones (base and cordless) can't be used at the same time. It is an either/or situation. Also the cordless handset can only pick up calls put on hold on the base they are attached to and not other 25414RE3 or 25415RE3 phones.I do not recommend this 4 line cordless phone even though it is very reasonably priced.