When attempting to transfer to an external number from inside the Auto Attendant, you get an immediate busy signal, known as a "fast busy." Inbound calls can still be received and are not affected.
Re-save settings in Account Manager, making sure a Main Number has been entered and selected.
- Log in to Account Manager.
- Click Phone System > Company Settings.
- Select a Main Number, if not yet selected.
- At the bottom of the page, click Save.
The source of this issue is that the Main Number for the account has not been properly set. This can stem from various issues: no company number has been selected; the previous company number was a line item that was cancelled; or the DID was changed or phone numbers on the account were moved around.