People who mumble, talk very quietly, or otherwise don't direct their voice to a mic; no one on the phone can hear them. Or those who rustle stuff right by the mic, so nobody on the phone can hear anything. How many times is it polite to interrupt and say, "Hey, speak up!"?

People who don't mute their phones when they're in a noisy environment, so the entire team learns which plane is boarding but cannot hear each other speak.

People who refer to a lot of visuals without bothering to send the file to telecommuters; and then they don't refer aloud to the data they're pointing at.

Meeting organizers who forget to conference in the remote people for the meeting.

And that's the minor problems.