Cannot call a cancelled number from a voipo number

Can anyone tell me why, after cancelling an account, and the former customer has ported his number to a different provider, when you call the cancelled number from a voipo number you get a busy signal, but when called from an off network phone, it will ring through? Does this mean that a current customer cannot ever call a cancelled number from his current voipo number, even though the number was transferred out of voipo's network?

It may still be lingering in our system so our system routing it internally.

Open a ticket with the number and we can make sure it's fully removed.