Richard F. has been busy faxing and picking up phones to try to answer this one. I think he's got it, the answer that is.

The engineers who designed the fax machines made them do that on purpose. Here's why. Fax machines send pictures to each other by sending special sounds. They sound like whistles, beeps and buzzes. These sounds are sent over the telephone lines by electricity, just like the sounds you make when you talk into the phone. The sending machine's sounds are in a special code. This code stands for the picture the fax machine is sending. The receiving fax machine also sends special sound codes back, to let the sender know if anything goes wrong.

When you pick up the phone, this makes other noises on the phone line. If the receiving machine kept on working, it would print a mixed-up mess instead of the picture it was supposed to print. The sender might not even know anything was wrong. To prevent that from happening, the machines are designed to cut off whenever anything goes wrong. That way the person receiving the message knows he only got the information he was supposed to, and not some crazy mixed-up message. And, the person sending the fax knows that something went wrong and she has to send it again. That's why you can't use the phone when the fax machine is busy.