2 line telephone patcher / switcher .. help please

Does any one know of a circuit that would do this.

Say you have 2 seperate tel lines A and B, if I call line A from an outside line the circuit will lift and give me a dial tone on line B so i can dial a number. So basically its like a call from line A will lift line B and I can start dialling using line B dialtone. ( does that make sense?)

Anybody?

Thanks in advance.

Roy

So what happens to line A? Does it keep on ringing until the caller hangs up?