I was a TSPS (dial 0 or 0+) operator in the late 1970s and it was very possible. If I remember correctly we key pulsed 700 + the subject phone number and it would cut right in on the line. We could hear the conversation of both parties. The purpose was for emergency interrupts. This would be limited to phones that we served as local operators. (Charlotte & most of Western NC) I could not be used for anything outside the area. Of course this was done on official telco equipment. Whether it could be accessed by the phreakers with a blue box is beyond me.