TL;DR
I forgot the birthday of my best friend. Later, when I called him to wish him a Happy Birthday. He is refusing my phone calls. I think he is angry at me. How can I convince him to forgive me?
Background
Today is my friend's birthday. We have been best friends since childhood and we share everything with each other. We used to talk on phone daily about almost every topic. We also meet once every 3 to 4 months because he lives 65 km away from my city. I remembered his Birthday before, but today I forgot to wish him a Happy Birthday. When he called me in the morning, I was talking with him about other stuff and did not remember. A few hours later, I opened my Facebook account and there I saw the notification about his birthday. I called him immediately, but he has refused my phone call. (Generally, he doesn't refuse my calls).
Clarifications
- I have wished him Happy Birthday by sending an SMS (of 'Happy Birthday') on his mobile.
- I can send another on Facebook messenger but he knows that I don't use it. Even if I use it then it will also not be enough for this special day.
- I can not travel to his place today.
- I cannot repeatedly call him on his mobile because he might be busy with his work. It may be possible in the evening but I don't know whether he will pick up or not.
Issue
I want to say I am sorry about forgetting his birthday and want to wish him a Happy Birthday. How can I communicate with him so that he will forgive me?
Note - Answer would be appreciated from any cultural background.