This only works if you're updating a handset from your Boost account online. (Not sure it will work if you update to your old phone then re-update to your current device.) Log in to your Boost account and click "Update handset". Enter the new MEID and go through the steps. At the last step (step 3), you'll see the following: MSL, MSID, and MDN (which is your own phone number.) There you go, you now have MSL! I have mine!