The answer to your question - what can someone do with my imei number is -
IMHO the IMEI is a device identifier (International Mobile Station Equipment Identity). You can retrieve it from any device by dialing *#06#. So technically it's not a real secret, and anyone borrowing your phone for a short moment can crack it. The IMEI is bound to the device, not your phone number or account. It could be used to track stolen phones, and to lock them out from networks (done in a few countries, but not across borders or carriers normally, they could not agree on a global DB). However, some phones apparently allow changing the IMEI, which is of course a bad idea (why hardware vendors allow such things BTW, is it too hard to burn in a number of bytes and then make them read-only?) and people could overwrite it easily.
If WhatsApp would use the IMEI for personal identity I could not change my phone anymore, if my old one breaks. The entire idea of GSM was and is that you are not bound to the phone, rather you are just bound to the SIM card, and SIM is what is required to be protected. The SIM is your key to your telephone number, that's the reason why you gotta PIN code and why it locks you out after 3 failed attempts...
I don´t believe there is any identity theft possible by knowing telephone numbers and IMEI. You need to protect your SIM card, somehow hack it and retrieve the IMSI. This is what identifies you against the mobile provider. Read this: https://www.quora.com/What-is-the-difference-between-ICCID-IMSI-and-IMEI-numbers Even your telephone number does not need to be stored on the SIM. For example, my private data card-only SIM displays an invalid phone number when asked, but on my invoice I can see a real one, which even accepts SMS. What Can Someone Do With My IMEI Number?
There is some information that it holds. The primary reason that IMEI numbers are important is their ability to help track down and secure lost or stolen phones. With every connection your phone makes, your IMEI number is shared with the provider. This serves a few vital purposes. Firstly, it helps locate the phone and guide it to the local connections so that you connect to a nearby tower. This provides a secondary benefit that in the event of a lost or stolen phone, it’s easy to locate where the last connection was. This is often used to locate missing people and understand the calls that were made from their devices to track their movements.
Finally, if the location isn’t enough to find your lost phone, every IMEI number in the UK is connected to a nationwide database that allows providers to see if it has been marked as lost or stolen. When you report your phone, it will then be blacklisted so that if it’s brought to be unlocked or sold, it will be flagged. Mobicode bases their extensive MobiCHECK device checkup on an IMEI lookup that allows us to then apply this information to a wide range of datasets for a fully comprehensive analysis of a phone’s history.
