If it is inconvenient for a debtor to receive a debt collection call on Sunday, and the debtor has specifically told collection agents not to call on Sundays, then debt collectors are not legally allowed to call. Under the Fair Debt Collection Practices Act, debt collectors who call on Sunday after being advised not to can be held in violation of the law.
The key here is that the debt collector must first be told that Sundays are not convenient for the debtor. It is up to the debtor to let the collection agency know that Sunday phone calls are unacceptable. Debtors should document the date and time of the call when they advise the bill collector that Sundays (or other times) are inconvenient. If calls continue to come in on Sundays, the debtor can then take legal action against the debt collector.