Imagine that you are busy. Now imagine that the phone rings. It seems reasonable to assume that you would be less likely to answer the call at that point than if you were available. Phone calls, like any type of communication, are established via a negotiation between the initiator of the communication and the recipient. The person placing the call chooses when to place it, and the person receiving the call chooses whether to answer it. One way a busy person can partake in the negotiation is to ignore incoming calls. Surprisingly, however, it turns out that people are actually more likely to answer the phone when busy!
