[Your letterhead, if desired; if not, your return address]
[Date of letter—month, day, and year]
[Recipient's first and last names]
[Street or P.O. box address]
[City, State ZIP code]
Dear [recipient's name]:
I am truly sorry for the misunderstanding and argument that we had when we were talking over the phone the other day. I was being unfair and impatient toward you when you were explaining what was going on with your manager and co-workers, and I said some things to you that I now regret. Please forgive me for doing that. Our friendship is so very special to me, and I would never want to do anything that would jeopardize our closeness.
Can we get together to talk more and have dinner, maybe this weekend? Please call or email me so we can make some plans. As I said, I apologize from the bottom of my heart.
[Sender's first and last names]