I have been calling you for about 2 hours - Present Perfect Tense
I had been calling you for about 2 hours - Past Perfect Tense
When to use each of them?
If you finally get through to someone on the phone after trying for two hours, you might say something like "I've been calling you for about two hours. "Where have you been?"
We use past perfect to talk about something that happened before something else in the past.