Thought is the past participle of think so when you say "I thought I had explained during the call" you mean you are stating that this action has happened in the past and you "think" (thought) in the past. When you say "I think I had explained during the call" you mean you are stating that this action has happened in the past and you "think" in the present. Hope I have not confused you more.

A: "How should I do this?" B: "I thought I (had) explained during the call" In this conversation, B is puzzled by A's question. B has a memory of explaining during the call, but A's question shows that A probably didn't hear or understand. B is being polite, and saying "maybe I didn't explain well enough." It means "Before you asked this question, I believed I explained it well, but now I see I didn't explain it well. Let me try again." A: "How should I do this?" B: "I think I explained during the call." This could be explained 2 different ways. If B says it it with an aggressive tone of voice: B is saying that they KNOW they explained it well, and A did a bad job of listening. "I think I explained it during the call. Next time, listen more closely. Do it this way. Do you understand?! Don't forget again!" if it is said with a confused tone of voice: B is saying that they believe the sentence is correct, but really don't remember. A: "Have you explained the new rules to Judy yet?" B: "I think I explained them during the call yesterday, but I've been really busy, and I can't actually remember. I'll check with Judy, and see if I explained the rules to her or not."

i agree with this explanation.. 👍

"Thought," he thinks that he did, but later forgot, just as you say. While "I think" means: "I am thinking now".