If all you have is IMEI, the only way ahead is with the help of the customers original network provider.
Basically as soon as you switch your phone on, or keep it on, all the IMEI is doing is continually asking the nearest signal beacon is "Is it okay for me to be on the network". This communication between IMEI and the signal beacon will store that communication for the network provider to respond with 'yes' or 'no'.
All the network provider will be able to give you is 'yes' the phone is still being used, or was last used on '.......' date at this location. Location triangulation / bi-signals (if only two responding beacons) can be pretty shit though depending on density of area and distance of beacons from each other. GPS is by far the more accurate (but not always precise!) method through "Find my iPhone" et al.
The network has probably blocked the handset, but easy enough to Jailbreak it. However, reprogramming an IMEI is not easy and is more hassle than its worth.
A reminder to ALL to ensure 'Find my iPhone' or Device Locator third party software is on ALL our mobile devices !!
Edited by luv u long time , 06 December 2013 - 11:07 AM.