Check out how to port a Phone Number to Twilio here or how to move Twilio numbers from your client's Twilio account to your own Twilio account here.
Once it's ported, How to import an existing Twilio phone number into a HighLevel client account?
Update:
Now you can click the button MOVE NUMBERS using the yellow button on the middle right to move the phone number from the master account to the subaccount.
Choose Master account as the source account:
If you are not able to move the number using this tool, please check the error message here.