Please remember it is important to ask permission of the people involved before you use their personal correspondence in your work.
Format: in text citation
The differences between attending a grammar school and a secondary modern school have been described at length in recent years (Surname, Year).
Format: reference
Caller surname, initials. (Year) Telephone conversation with First Name Surname. Day Month.
Example: in text citation
The differences between attending a grammar school and a secondary modern school have been described at length in recent years (Smith, 2019)
Example: reference
Smith, J. (2019) Telephone conversation with Lucy Ladd. 10th February