An IMEI is a long number that belongs to one mobile phone, not to your SIM card or your phone number. The network uses it to know which physical phone is on the line. The tidy school sentence says the phone sends this number when you make a call; though actually, the network uses it more widely whenever the phone joins the system, and shops use it to check a phone and to block a stolen one. It is usually about 15 digits. You can often find it in the phone’s settings, printed in the battery area on older phones, or by dialling a short code. People say the letters one by one: I-M-E-I. It is a technical word, but phone-shop staff in Taiwan use it all the time.