It is always wise to read the official staff interpretations (a/k/a Commentary) in connection with the regulatory text.
Comment 9(a)(3)-4 to § 1002.9 of Regulation B says —
Quote
4. Manner of compliance. In complying with the notice provisions of the Act and regulation, creditors offering business credit may follow the rules governing consumer credit. Similarly, creditors may elect to treat all business credit the same (irrespective of revenue size) by providing notice in accordance with §1002.9(a)(3)(i).
As for notifying a business applicant orally, you need to have an enforced procedure that records the date, time, and circumstance of such conversations (phone, in person, etc.), and whether reasons were given in the notification. If an applicant who is not given reasons for the adverse action requests those reasons, your procedures should include an annotation of how and when that information is provided.