Set up your phone number
When an Android user is added, the contact name shows up in red with "Not registered with iMessage" as the error message. To fix this, the creator of the group message should create a brand new group that includes non-iMessage users. This will tell the group message to use SMS/MMS instead of iMessage.
to use your number for iMessage you will have to go to Settings->Messages and turn on iMessage. ->(your Apple ID card at the top)->Name, Phone Numbers, Email and press „Edit“ next to „Contactable At“ and add another number.
If you're looking to add your phone number to iMessage on your Mac computer, you can do so in a few quick steps using your iPhone and Mac. Simply enable iMessage and add your phone number on both your iPhone and Mac, and you'll begin receiving messages to both devices in no time.
Set up your phone number
Part 3: How to turn registered/added number on or off on Mac/iPad
to use your number for iMessage you will have to go to Settings->Messages and turn on iMessage. ->(your Apple ID card at the top)->Name, Phone Numbers, Email and press „Edit“ next to „Contactable At“ and add another number.
You may see this message on your iPod touch, iPad or Mac. You see this message because the recipient cannot accept iMessages because the recipient does not have an iPhone or does not have iMessage enabled.
Steps to check total registered mobile numbers on your name
Enable Text Forwarding If the problem is that only some of your devices are encountering the “phone number not registered with iMessage” error, you should make sure that you're logged in with your Apple ID across all of the Apple devices you use.
Click on the Messages section. Enter the Send & Receive section. If you see the “Use your Apple ID for iMessage” option and select it if that is not the case yet. Look for the “YOU CAN RECEIVE IMESSAGES TO AND REPLY FROM” section and make sure that your phone number is selected there.
On iPhone and iPad
You can't register someone else for iMessage. They need to do it for themselves from their device. Not sure what this means but you tap on the blue + button to bring up your contacts list and then select the contact from there.
Click on the Messages section. Enter the Send & Receive section. If you see the “Use your Apple ID for iMessage” option and select it if that is not the case yet. Look for the “YOU CAN RECEIVE IMESSAGES TO AND REPLY FROM” section and make sure that your phone number is selected there.