It is generally accepted that personal communications such as letters and telephone conversations do not need to be included in the final reference list. However, you can refer to them in an assignment and include the full details in your text.
In-Text Reference
Smith (2020) wrote..
Reference List Format
SENDER'S NAME, year. Email to RECIPIENT'S NAME. Day/Month of email
Reference List Example
SMITH, B., 2020. Email to K. JONES. 24th August
Letter
You should get permission to refer to the communication before using it.
In-Text Reference
As stated in a letter from D. White on 21st July 2007, it is...
Reference List Format
NAME(S), Year. [Outline of topic discussed] (personal communication, date)
Reference List Example
WHITE, D., 2007. [Letter on the history of education in Southampton] (personal communication, 21 July)
Telephone Call
You should get permission to refer to the communication before using it.
In-Text Reference
J. Brown outlined this idea in a telephone call on 10th August 2008.
Reference List Format
NAME(S), Year. [Outline of topic discussed] (personal communication, date)
Reference List Example
BROWN, J., 2008. [Telephone conversation on study skills] (personal communication, 10 August)