As you may know, when adding new inbound phone numbers in FastCall, the phone numbers must be provisioned by FastCall. If you have existing numbers in Twilio you email Twilio to ask to have the numbers moved to the FastCall subaccount. Once added to the FastCall subaccount, you will not be able to control these in Twilio (not that you would want to.) You will see a FastCall logo next to the number to show that the number is now controlled by FastCall. Good.
Bask in Salesforce, in the FastCall app, under Advanced Settings there is an option to sync FastCall with Twilio (resync actually). Hit this, and the transferred numbers will now appear in the Phone tab ready to be assigned.
In our post on inbound call routing we discuss this process and give a tip on porting numbers to Twilio. If you plan to port a number from another carrier to Twilio you may want to verify that your current carrier will release the number. Once confirmed, another good idea to to provision a test number first in FastCall. Better to test with a throw away number, then go through the transfer process only to find an issue that could have been discovered via a quick test.
OK, that said, use this form on Twilio to begin the transfer process.
You should also review this Twilio FAQ on number porting.