Key in *#06#

1. Turn off your handset.
2. Turn it on again.
3. Allow the handset to load completely.
4. Press the following keys one by one: * # 0 6 #
5. Read the screen. As you press on the last #, a number will appear with the title IMEI number

[…]

If your mobile phone is stolen, call or go to the nearest service center of your mobile network, and give that IMEI number to staff and ask him to block that number for a period of time, so no one can use your mobile phone for any illegal purpose.

Source: WikiHow

This worked on my phone without having to turn if off and on again.