I can remember at my first job (which was a long way away from the Exchange / central office). The carrier signal was a little weak and I had to whistle the co

I can remember at my first job (which was a long way away from the Exchange / central office). The carrier signal was a little weak and I had to whistle the correct tone down the line to get the other end to pickup :-) We also when we had a project that used modems for remote data collection our electronics shop modified (technically illegal) the modems with adjustable gain