In order to route inbound calls with Fastcall, your phone numbers must be controlled by Twilio. You will need to provision net new numbers with Twilio, or port your existing numbers to Twilio. If adding new numbers, the phone numbers must be provisioned by Fastcall to use the number in Fastcall. If you have existing numbers in Twilio you will need to ask Twilio to move these numbers to the Fastcall subaccount.
TIP: 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.