Yes. If the message is unable to be delivered to the recipient via iMessage, then it is sent as SMS instead. So for example if you try to send a message to someone and their phone is turned off or doesn’t have signal, it will eventually fail and then send as SMS.
This is all controlled via a setting in iOS iMessage settings “Send as SMS”, which I believe is turned on by default but I’m not sure. So your mileage may vary.
