You did not state exactly what equipment you were using for each side of the line (or how it works for that matter). I assumed both the talk and dial mechanism were standard butt sets that have a monitor mode switch. If that is true, then the circuit I proposed will work. In monitor mode, insufficient current will flow thru the butt sets to complete the circuit and take the line off hook, untill you switch it to talk/dial mode. Let me know it that is or is not true/ then I can help further.
Dialtone