Is there a requirement where we must disclose a telephone number or mailing address that consumers may use to contact the bank or its third-party servicers regarding complaints they may have?

Not unless there might be a State law. Many States have specific disclosure requirements informing customers of which State agency to contact to submit complaints, especially for mortgage lending. There are also various regulations that require disclosure of contact information to report suspected errors. But complaints specifically to the bank - not that I am aware of at the Federal level.

but you do need to make sure you have a process at the bank so if a formal complaint is called in to someone at the bank, it is logged and responded to within the appropriate timeframe. which is why many banks have a designated number to call.