"Я міг передзвонити, але я забув."

Translation: I could have called back but I forgot.

[Stacymitch]

"i could have called back, but i forgot" works also

[Ally_October]

I agree, but I'd go further and say 'could have' is better English and therefore should be the first choice.

[Mr.Xee]

I could call back, buy I had forgotten

[retired82844]

I could call back, but I forgot.