Stolen Devices with SIMs - What TomTom can and can't do
See Everything You Should Know About Your IMEI Number
Also from How to track a lost device with no anti-theft software installed
"Every time a device logs into a particular network to make or receive calls, its IMEI number is automatically emitted and tracked. This gets registered with the service provider. […] The police has a database of devices which are in white, grey and black lists, so a device which is in the black list is known to be stolen. Once it’s reported stolen by you, the IMEI number is blacklisted across the country. If an IMEI device on the black list is used to make a call, the police will get to know about it immediately. There is a chance that they will track the device if it comes up for resale or reset."
But all this relies on TomTom releasing the IMEI number to the user who can then give this information to the police. So if you have your device stolen don't forget when contacting TT to report it stolen, (and thus block any further updates/services,) to ask for the IMEI number.