past tense vs past perfect

[Mitsaboshu]

scenario:
I forgot to call you last night so I call you explaining to you the reason on phone now.

case 1:
on the phone, I said:
Sorry, I forgot calling you last night.

case 2:
on the phone, I said:
Sorry, I had forgotten calling you.

In case 1, the past tense is correctly used because of the "last night"
How about case 2. Is it right?
Waiting for your urgent help, thanks.

[Raymott]

scenario:
I forgot to call you last night so I call you explaining to you the reason on phone now.
I forgot to call you last night so I'm calling you now to explain the reason I wanted to speak with you.

case 1:
on the phone, I said:
Sorry, I forgot calling you last night.
Sorry, I forgot to call you last night.

case 2:
on the phone, I said:
Sorry, I had forgotten calling to call you.

In case 1, the past tense is correctly used because of the "last night"
How about case 2. Is it right?
Waiting for your urgent help, thanks.

No. 2. isn't right. You're referring to one event in the past. You only use the past perfect if you have two events in the past, one occurring before the other.

This is a correct use of the past perfect and simple past tense:
Me: Why did you call the person today?
You: I called her this morning [simple past: most recent thing in the past] because I had forgotten [past perfect: 2nd thing - further back in the past]to call last night.

Note: you could use the simple past twice here, because the context makes the meaning clear.