Interconnection of telephone lines

Hi all,
I’m new to this forum and chanced upon this while searching for one to find a solution to my requirement.
Apart from the normal PSTN phone I’ve another phone provided by my Company which operates only within our township/workplace and is not accessible to/from the public telephone network.
I’m looking for some sort of an interfacing gadget(I do not know the technical term for this..)to which we can connect both the lines(like in a 2 line cord-less phone)so that callers from the public network can dial into the first line and get answered automatically and be able to get the dialtone of the second line..so that they can dial my workplace number over the second network and get through to me.Similarly I would like to be able to dial to my house and get connected to the public network line so as to be able to contact the public network phone numbers.I want this to happen automatically without someone being present at the instrument that interfaces both the lines(that’s at my home) by dialing a sort of extension code like we do for PBX extensions..Can you please suggest what I should buy to accomplish this..Thanking you in advance