A full Salesforce sandbox is intended to be used as a testing environment and is a replica of your Salesforce production org. However, Fastcall will not replicate from your sbox to your production org. The Salesforce site has a different URL. A unique Twilio subaccount will be created in production and this cannot conflict with a sbox. If you are installing Fastcall for the first time in the sbox, the Twilio subaccount created will be unique. If you in installed Fastcall in production first, and the sbox is a clone of production, the Twilio connection will be the same as production. Which is fine for basic testing. If you need a unique Twilio connection you will need to uninstall Fastcall from the sbox and install a clean version of Fastcall. This will create a unique Twilio subaccount.