You need a 40db pad to reduce the line level to mic level. I use one the other way around to plug into my video camera mic input from the headphone output to record the intercom on the camera.
Try a company called Canford audio based in Tyne & Wear or Maplin might help, explain to them you need to reduce line level to mic level and they will know what you mean and put you right.
Be very careful not to blow your intercom with too much level into a mic input. The pad should prevent that, but test it fully on the ground first, it would be a silly way to go 7600.
Not sure I would like the distraction myself but I guess you are a grown up.
Hope that helps!!
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