If the person ever had their number attached to an iPhone and they never reset the phone and removed from icloud it will try to use imessages. They need to delete the phone if they have an icloud account.
You have to send it as a regular SMS not as an iMessage
They should be able to receive your messages, but you won't be able to see if they read your messages since they have Android. You can only see if a message is read on Apple to Apple products.
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