@cheribaker your iPhone using friends may need to remove your phone number from their contact for you completely and then re-add it. They might also need to make sure that “Send as SMS" and "MMS Messaging" is enabled in their phone/iPad settings.
Once those options are enabled then the very least, they should be able to force-send the messages to you by SMS once the iMessage (blue) message fails. At which point it usually defaults to the SMS (green)
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How to Force Send Text Message Instead of iMessage to a Contact
Do you wish to force send an iMessage as a Text Message to your contact? This post will show you all the steps to do exactly that.
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