Final answer:
The most common form of substantive communication between the tax professional and client is a telephone call or e-mail.
Step-by-step explanation:
The most common form of substantive communication between the tax professional and his or her client is a telephone call or e-mail. These forms of communication allow for direct and immediate interaction, making it easier for the tax professional to address any concerns or questions the client may have. Additionally, telephone calls and e-mails provide a convenient way for documents and information to be exchanged between the tax professional and client.