iMessage is an Apple messaging service. Don’t forget to turn it off, or SMS/MMS messages may continue to be directed to your old iPhone or iPad instead of being routed to your new Android phone.
Turn off iMessage before you remove the SIM on your iPhone or iPad by going to "Settings", then "Messages". Finally, set iMessage to “off”.
If you no longer have your old iPhone, you can request that Apple deregister your phone number with iMessage. Learn more.