Can we forbid people from talking to us when we are having a conversation on the phone? These people become frenzied as they assume what the person on the other line is saying and begin talking over you, telling you what to say, and trying to answer their own imaginary conversations.
It is nearly impossible to hold two conversations at once. If the telephone conversation involves those in the room, say with directions for the group or a suggestion to meet at a restaurant, only the person on the phone should be engaged in the conversation. It is possible to put the caller on hold and address your present company, who should be waiting patiently.
This form of interrupting is incredibly rude and frustrating. When you encounter it, first hold up your hand to signal the interrupter to stop. If they persist, place the other person on hold while you explain to the interrupter that they are interrupting your phone conversation. They may have valid points to make but their way of getting them across is ill-timed.