1. Is the number you want to import a Twilio number?
Yes - go to 2.
No - Check out how to port a Phone Number to Twilio here
2. Once it's ported as a Twilio number, How to import an existing Twilio phone number into a HighLevel client account?
How to locate the gaining SID for the account:
1. Click on sub-account name on the top left
2. Click on Switch to Agency View
3. Click on Settings
4. Click on Phone Integration
5. That's the master Twilio account SID
6. Click on Sub Account Settings
7. Click on the pencil icon on the right for the sub-account you want to move the number to
8. That'd be the gaining Account SID for the sub-account you want to move the number to
Click on Update Credential to update the Twilio Account SID and auth token for the sub-account