They were registering the number and iCloud account, that’s how it works.

They just built their own ANP service to interface with GCM.

I tried it, it used my existing iCloud account to send messages. Could do that if it didn’t connect to iCloud and get the RSA key.

As a matter of fact, to stop using it you have to de-register your phone number from iCloud.

If you read the apps or devs docs it’s all explained.