1.) Talking to the air. Give a name and face to the conference-call device (I call mine the “cowpat”). Talk to it as though it were a person sitting in the room. People inevitably forget that you aren't in the room and begin having sidebars, talking in hushed tones, etc. If you pretend that the cowpat is a person, you will make the conversation more effective for all parties.
2.) Multitasking. It is so easy to put the phone on mute to mask the typing of your fingers on the keyboard. It is hard enough as it is to focus on all the voices and all the distractions. Shut your computer down, turn away from the desktop, or go to another part of the office. It is hard enough to follow a teleconference without multitasking. If you have to multitask, reschedule the call.
3.) Getting too comfortable. If you are taking a teleconference from home at an odd or late hour, keep it professional. Do not get comfortable, get in bed, get a snack, or get sleepy. Make a section of your house the "conference room" for the hour.
4.) Interrupting. If there are many of you in one place, practice not speaking over each other or having sidebars. In fact, have someone chair the call and moderate the group. Good points are lost otherwise, and it is impossible to follow.
5.) Using your cell phone without telling the group. Announce what device you're using. If you are on a cell phone, tell people so they can expect your call to drop or that you will be in a loud place. If you are in said loud place, do use your mute button. Again, find a place and call it your conference room.
6.) Forgetting the phone is a microphone. And it picks up the shuffling of your papers, the clinking of your coffee cup, the dragging of the phone/cowpat, and as a result, it makes it very unpleasant for the listener. And, in this day and age of the handheld device, do not put your cell phone near the cowpat. As your phone reaches into the cloud for your e-mails, the transmission interference is heard on the line.