Which tense can complete this sentence: "What _______ when I called you"?

At first, it is important to note that, the action took place in the past, as such, the verb entering in that space must agree with the verb "called", which is in the past. Also "were" plural verb instead of "was", should equally agree with "you" (second person pronoun).

The word called in the sentence is past tense and were is the appropriate word since they are both past tense .