I didn't realize you had called, that's why I was late getting back to you. That was bad of me.
この場合、次のような言い方ができますよ。
ーI didn't realize you had called, that's why I was late getting back to you. That was bad of me.
「あなたからの着信に気が付かなくて、返事が遅くなった。感じ悪くてごめんね」
didn't realize you had called で「あなたが電話くれたことに気がつかなった」=「着信に気が付かなかった」と言えます。
to get back to ... で「…に返事する・後で連絡する」
that was bad of me で「私は悪かった」=「私って酷いよね・感じ悪かったよね」と言うニュアンスになります。
ご参考まで!